Define a macro to indicate if the compiler is actually GCC.

Pretty much all new compilers these days will define all the GCC
macros so it's impossible to detect GCC from predefined macros
without adding a list of all the compilers that pretend to be GCC
if defined(__GNUC__) && !defined(__clang__) && !defined(____INTEL_COMPILER)

src/build/Common.gmake

@@ -314,6 +314,9 @@ CCPRE := $(lastword $(subst /, ,$(CCPRE)))
 ifeq ($(CCPRE),clang)
  CFLAGS += -Wno-invalid-source-encoding
 endif
+ifeq ($(CCPRE),gcc)
+ CFLAGS += -DREALLY_GCC
+endif
 LIBODIR :=	$(CCPRE).$(machine).lib.$(BUILDPATH)
 OBJODIR :=	$(CCPRE).$(machine).obj.$(BUILDPATH)
 MTOBJODIR :=	$(CCPRE).$(machine).obj.$(BUILDPATH)-mt