force --std=gnu89 for clang (and gcc), properly

This implements 3a4860a7 in the proper place.

Change-Id: I7b60602933819561d0bff5e72dcd5ee8986f63c0

solenv/gbuild/platform/com_GCC_defs.mk

@@ -107,6 +107,10 @@ gb_CXX03FLAGS := -std=gnu++98 -pedantic-errors -Wno-long-long \
     -Wno-variadic-macros -Wno-non-virtual-dtor -Wno-deprecated-declarations
 endif
 
+# On Windows MSVC only supports C90 so force gnu89 (especially in clang) to
+# to catch potential gnu89/C90 incompatibilities locally.
+gb_CFLAGS_COMMON += -std=gnu89
+
 ifeq ($(ENABLE_LTO),TRUE)
 ifeq ($(COM_GCC_IS_CLANG),TRUE)
 gb_LTOFLAGS := -flto