diff --git a/Makefile.in b/Makefile.in
index 2226b7429..d1ceee496 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -57,7 +57,7 @@ clang_plugin: clang_setup
 ifeq (@IS_RELEASE_TREE@,no)
 	$(MAKE) -C $(FCP_DIR)/libtooling all \
 	  CC=@CC@ CXX=@CXX@ \
-	  CXX="@CXX@" CXXFLAGS="@CXXFLAGS@" \
+	  CFLAGS="@CFLAGS@" CXXFLAGS="@CXXFLAGS@" \
 	  CPP="@CPP@" LDFLAGS="@LDFLAGS@" LIBS="@LIBS@" \
 	  LOCAL_CLANG=@CLANG_PREFIX@/bin/clang \
 	  CLANG_PREFIX=@CLANG_PREFIX@ \
@@ -65,7 +65,7 @@ ifeq (@IS_RELEASE_TREE@,no)
 	$(MAKE) -C $(FCP_DIR)/clang-ocaml all \
           build/clang_ast_proj.ml build/clang_ast_proj.mli \
 	  CC=@CC@ CXX=@CXX@ \
-	  CXX="@CXX@" CXXFLAGS="@CXXFLAGS@" \
+	  CFLAGS="@CFLAGS@" CXXFLAGS="@CXXFLAGS@" \
 	  CPP="@CPP@" LDFLAGS="@LDFLAGS@" LIBS="@LIBS@" \
 	  LOCAL_CLANG=@CLANG_PREFIX@/bin/clang \
 	  CLANG_PREFIX=@CLANG_PREFIX@ \