FREETYPE_CFLAGS is already set when using external freetype

When you're on Debian derivatives, using internal cairo but external
(system's) freetype, the "ft2build.h" is in /usr/include/freetype2/,
not /usr/include/, which is already in FREETYPE_CFLAGS since configure.ac

Change-Id: I24c697776bd77d423ebeafd38315cb76cad28f9b
Reviewed-on: https://gerrit.libreoffice.org/23334Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>

external/cairo/ExternalProject_cairo.mk

@@ -45,7 +45,7 @@ $(call gb_ExternalProject_get_state_target,cairo,build) :
 		ZLIB3RDLIB="$(if $(SYSTEM_ZLIB),,-L$(WORKDIR)/LinkTarget/StaticLibrary) -lz" \
 		png_REQUIRES="trick_configure_into_using_png_CFLAGS_and_LIBS" \
 		png_CFLAGS="$(LIBPNG_CFLAGS)" png_LIBS="$(LIBPNG_LIBS)" \
-		FREETYPE_CFLAGS="-I$(call gb_UnpackedTarball_get_dir,freetype)/include" \
+		$(if $(SYSTEM_FREETYPE),,FREETYPE_CFLAGS="-I$(call gb_UnpackedTarball_get_dir,freetype)/include") \
 		FONTCONFIG_CFLAGS="-I$(call gb_UnpackedTarball_get_dir,fontconfig)" \
 		$(if $(filter IOS,$(OS)),--disable-shared,--disable-static) \
 		$(if $(filter ANDROID,$(OS)),--disable-shared,--disable-static) \