[make] add LIBGCC to the global config

This forces the system to rebuild if the compiler's path changed, which
probably means it is a version switch.

engine.mk

@@ -334,6 +334,7 @@ GLOBAL_DEFINES += ARCH_CPPFLAGS=\"$(subst $(SPACE),_,$(ARCH_CPPFLAGS))\"
 GLOBAL_DEFINES += ARCH_ASMFLAGS=\"$(subst $(SPACE),_,$(ARCH_ASMFLAGS))\"
 GLOBAL_DEFINES += ARCH_LDFLAGS=\"$(subst $(SPACE),_,$(ARCH_LDFLAGS))\"
 GLOBAL_DEFINES += TOOLCHAIN_PREFIX=\"$(subst $(SPACE),_,$(TOOLCHAIN_PREFIX))\"
+GLOBAL_DEFINES += LIBGCC=\"$(subst $(SPACE),_,$(LIBGCC))\"
 
 ifneq ($(OBJS),)
 $(warning OBJS=$(OBJS))