diff --git a/Makefile.lib b/Makefile.lib
index 6c59c75969..4df40c9d2c 100644
--- a/Makefile.lib
+++ b/Makefile.lib
@@ -24,10 +24,10 @@ dist_files :=
 QUIET = @
 
 %.o: %.cc
-	$(QUIET) $(CXX) -o $@ -c $< -g -fPIC $(CXXFLAGS) $($@_CXXFLAGS) -MMD -MF $(basename $@).dep -MP
+	$(QUIET) $(CXX) -o $@ -c $< -g -fPIC $(GLOBAL_CXXFLAGS) $(CXXFLAGS) $($@_CXXFLAGS) -MMD -MF $(basename $@).dep -MP
 
 %.o: %.c
-	$(QUIET) $(CC) -o $@ -c $< -g -fPIC $(CFLAGS) $($@_CFLAGS) -MMD -MF $(basename $@).dep -MP
+	$(QUIET) $(CC) -o $@ -c $< -g -fPIC $(GLOBAL_CFLAGS) $(CFLAGS) $($@_CFLAGS) -MMD -MF $(basename $@).dep -MP
 
 
 # Generate Make rules for libraries.
diff --git a/Makefile.new b/Makefile.new
index af2c4cec3c..1ae88d42fe 100644
--- a/Makefile.new
+++ b/Makefile.new
@@ -10,6 +10,7 @@ SUBS = \
   src/nix-env/Makefile.new \
   src/nix-daemon/Makefile.new
 
+GLOBAL_CXXFLAGS = -I . -I src -I src/libutil -I src/libstore -I src/libmain -I src/libexpr
+
 include Makefile.lib
 
-CXXFLAGS += -I . -I src -I src/libutil -I src/libstore -I src/libmain -I src/libexpr -O0