Add Windows cmark include fallback

Author: danjboyd

ObjcMarkdown/GNUmakefile

@@ -13,6 +13,9 @@ ObjcMarkdown_CPPFLAGS += $(CMARK_CFLAGS) -DOBJCMARKDOWN_ENABLE_TOML_THEME=$(OBJC
 ifneq (,$(findstring mingw,$(GNUSTEP_HOST_OS)))
 ObjcMarkdown_OBJCFLAGS += -include sys/types.h "-Dmode_t=unsigned short"
 ObjcMarkdown_LIBRARIES_DEPEND_UPON += -lgdiplus
+ifeq ($(strip $(CMARK_CFLAGS)),)
+CMARK_CFLAGS := -I/clang64/include
+endif
 endif
 ifeq ($(OBJCMARKDOWN_ENABLE_TOML_THEME),1)
 ObjcMarkdown_C_FILES = ../third_party/tomlc99/toml.c