Rework dylibbundler test

- Add support for a sequence of guard predicates for a test, rather than
  limiting to a single predicate.
- Use that support to skip the dylibbundler test when
  user_settings.macos_add_frameworks makes the test unnecessary.
- Use StaticSubprocess to avoid running dylibbundler repeatedly if
  building multiple targets for which the test is necessary.
- Capture stderr from the child process, and log it, instead of letting
  it be written directly to the stderr inherited from the caller of
  SCons.
- On failure, write to the SConf log the return code of dylibbundler,
  its stdout, and its stderr.  This may help users diagnose the problem
  if it is more complicated than the tool not being installed.
- On failure, show a more direct suggestion about how to avoid needing
  dylibbundler.

SConstruct

@@ -222,9 +222,18 @@ class _ConfigureTests:
 	class Collector:
 		class RecordedTest:
 			__slots__ = ('desc', 'name', 'predicate')
-			def __init__(self,name,desc,predicate=None):
+			def __init__(self,name,desc,predicate=()):
 				self.name = name
 				self.desc = desc
+				# predicate is a sequence of zero-or-more functions that
+				# take a UserSettings object as the only argument.
+				# A recorded test is only passed to the SConf logic if
+				# at most zero predicates return a false-like value.
+				#
+				# Callers use this to exclude from execution tests which
+				# make no sense in the current environment, such as a
+				# Windows-specific test when building for a
+				# Linux target.
 				self.predicate = predicate
 			def __repr__(self):
 				return '_ConfigureTests.Collector.RecordedTest(%r, %r, %r)' % (self.name, self.desc, self.predicate)
@@ -244,16 +253,28 @@ class _ConfigureTests:
 class ConfigureTests(_ConfigureTests):
 	class Collector(_ConfigureTests.Collector):
 		def __init__(self):
+			# An unguarded collector maintains a list of tests, and adds
+			# to the list as the decorator is called on individual
+			# functions.
 			self.tests = tests = []
 			super().__init__(tests.append)
 
 	class GuardedCollector(_ConfigureTests.Collector):
-		__RecordedTest = _ConfigureTests.Collector.RecordedTest
 		def __init__(self,collector,guard):
+			# A guarded collector delegates to the list maintained by
+			# the input collector, instead of keeping a list of its own.
 			super().__init__(collector.record)
-			self.__guard = guard
-		def RecordedTest(self,name,desc):
-			return self.__RecordedTest(name, desc, self.__guard)
+			# `guard` is a single function, but the predicate handling
+			# expects a sequence of functions, so store a single-element
+			# tuple.
+			self.__guard = guard,
+			self.__RecordedTest = collector.RecordedTest
+		def RecordedTest(self, name, desc, predicate=()):
+			# Record a test with both the guard of this GuardedCollector
+			# and any guards from the input collector objects, which may
+			# in turn be instances of GuardedCollector with predicates
+			# of their own.
+			return self.__RecordedTest(name, desc, self.__guard + predicate)
 
 	class CxxRequiredFeature:
 		__slots__ = ('main', 'name', 'text')
@@ -604,7 +625,7 @@ struct %(N)s {
 		# When LTO is used, the optimizer is deferred to link time.
 		# Force all tests to be Link tests when LTO is enabled.
 		self.Compile = self.Link if user_settings.lto else self._Compile
-		self.custom_tests = [t for t in self.custom_tests if t.predicate is None or t.predicate(user_settings)]
+		self.custom_tests = [t for t in self.custom_tests if all(predicate(user_settings) for predicate in t.predicate)]
 	def _quote_macro_value(v):
 		return v.strip().replace('\n', ' \\\n')
 	def _check_sconf_forced(self,calling_function):
@@ -2900,18 +2921,34 @@ BOOST_AUTO_TEST_CASE(f)
 	# however it's only meaningful for macOS builds, and, for those,
 	# only required when frameworks are not used for the build.  Builds
 	# should not fail on other operating system targets if it's absent.
-	@_guarded_test_darwin
-	def _check_dylibbundler(self,context):
-		context.Display('%s: checking whether dylibbundler is installed...' % self.msgprefix)
-		if self.user_settings.macos_add_frameworks:
-			context.Result('n/a')
+	@GuardedCollector(_guarded_test_darwin, lambda user_settings: not user_settings.macos_add_frameworks)
+	def _check_dylibbundler(self, context, _common_error_text='; dylibbundler is required for compilation for a macOS target when not using frameworks.  Set macos_add_frameworks=False (and handle the libraries manually) or install dylibbundler.'):
+		context.Display('%s: checking whether dylibbundler is installed and accepts -h...' % self.msgprefix)
+		try:
+			p = StaticSubprocess.pcall(('dylibbundler', '-h'), stderr=subprocess.PIPE)
+		except FileNotFoundError as e:
+			context.Result('no; %s' % (e,))
+			raise SCons.Errors.StopError('dylibbundler not found%s' % (_common_error_text,))
+		expected = b'dylibbundler is a utility'
+		first_output_line = p.out.splitlines()
+		if first_output_line:
+			first_output_line = first_output_line[0]
+		# This test allows the expected text to appear anywhere in the
+		# output.  Only the first line of output will be shown to SCons'
+		# stdout.  The full output will be written to the SConf log.
+		if p.returncode:
+			reason = 'successful exit, but return code was %d' % p.returncode
+		elif expected not in p.out:
+			reason = 'output to contain %r, but first line of output was: %r' % (expected.decode(), first_output_line)
+		else:
+			context.Result('yes; %s' % (first_output_line,))
 			return
-		dylibbundler_output = os.popen('dylibbundler -h').read()
-		if 'dylibbundler is a utility' not in dylibbundler_output:
-			context.Result('no')
-			raise SCons.Errors.StopError('dylibbundler is required for compilation for a macOS target when not using frameworks')
-		context.Result('yes')
-		return
+		context.Result('no; expected %s' % reason)
+		context.Log('''scons: dylibbundler return code: %r
+scons: dylibbundler stdout: %r
+scons: dylibbundler stderr: %r
+'''  % (p.returncode, p.out, p.err))
+		raise SCons.Errors.StopError('`dylibbundler -h` failed to return expected output; dylibbundler is required for compilation for a macOS target when not using frameworks.  Set macos_add_frameworks=False (and handle the libraries manually) or install dylibbundler.')
 
 	# This must be the last custom test.  It does not test the environment,
 	# but is responsible for reversing test-environment-specific changes made