Resolves: fdo#61571 cairo not linking against internal libpng

the flag is called SYSTEM_LIBPNG not SYSTEM_PNG

Change-Id: I703a669f544e539f7fcfa64c0121a73f0608aecc
(cherry picked from commit c0f85e68)
Reviewed-on: https://gerrit.libreoffice.org/2462Reviewed-by: Niko Rönkkö <ronkko@iki.fi>
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>

cairo/ExternalProject_cairo.mk

@@ -40,7 +40,7 @@ $(call gb_ExternalProject_get_state_target,cairo,build) :
 		pixman_LIBS="-L$(call gb_UnpackedTarball_get_dir,pixman)/pixman/.libs -lpixman-1" \
 		COMPRESS=$(if $(filter YES,$(SYSTEM_ZLIB)),compress,z_compress) \
 		ZLIB3RDLIB=-lz \
-		$(if $(filter NO,$(SYSTEM_PNG)),png_CFLAGS="-I$(OUTDIR)/inc/external/libpng" png_LIBS="-L$(OUTDIR)/lib -lpng") \
+		$(if $(filter NO,$(SYSTEM_LIBPNG)),png_CFLAGS="-I$(OUTDIR)/inc/external/libpng" png_LIBS="-L$(OUTDIR)/lib -lpng") \
 		$(if $(filter IOS,$(OS)),--disable-shared,--disable-static) \
 		$(if $(filter ANDROID IOS,$(OS)),--disable-xlib,--enable-xlib) \
 		$(if $(filter IOS,$(OS)),--enable-quartz --enable-quartz-font) \