gnu: mesa: Stay on RTTI-less LLVM.

* gnu/packages/llvm.scm (llvm-without-rtti): New public variable.
* gnu/packages/gl.scm (mesa)[inputs]: Use that instead of LLVM.

gnu/packages/gl.scm

@@ -259,7 +259,8 @@ (define-public mesa
         ("libxvmc" ,libxvmc)
         ,@(match (%current-system)
             ((or "x86_64-linux" "i686-linux")
-             `(("llvm" ,llvm)))
+             ;; FIXME: Change to 'llvm' in the next rebuild cycle.
+             `(("llvm" ,llvm-without-rtti)))
             (_
              `()))
         ("makedepend" ,makedepend)

gnu/packages/llvm.scm

@@ -91,6 +91,26 @@ (define-public llvm
 of programming tools as well as libraries with equivalent functionality.")
     (license license:ncsa)))
 
+;; FIXME: This package is here to prevent many rebuilds on x86_64 and i686
+;; from commit fc9dbf41311d99d0fd8befc789ea7c0e35911890.  Update users of
+;; this in the next rebuild cycle.
+(define-public llvm-without-rtti
+  (package
+    (inherit llvm)
+    (arguments
+     `(#:configure-flags '("-DCMAKE_SKIP_BUILD_RPATH=FALSE"
+                           "-DCMAKE_BUILD_WITH_INSTALL_RPATH=FALSE"
+                           "-DBUILD_SHARED_LIBS:BOOL=TRUE"
+                           "-DLLVM_ENABLE_FFI:BOOL=TRUE"
+                           "-DLLVM_INSTALL_UTILS=ON")
+       #:build-type "Release"
+       #:phases (modify-phases %standard-phases
+                  (add-before 'build 'shared-lib-workaround
+                    (lambda _
+                      (setenv "LD_LIBRARY_PATH"
+                              (string-append (getcwd) "/lib"))
+                      #t)))))))
+
 (define* (clang-runtime-from-llvm llvm hash
                                   #:optional (patches '()))
   (package