fix compilation of pngformats.c under FreeMint

src/fileformats.c

@@ -27,9 +27,6 @@
 ///@file fileformats.c
 /// Saving and loading different picture formats.
 
-#ifdef __MINT__
-#undef _GNU_SOURCE
-#endif
 #include <string.h>
 #ifndef __no_pnglib__
 // just for png_sig_cmp()

src/pngformat.c

@@ -28,6 +28,9 @@
 /// Saving and loading of PNG file format
 
 #ifndef __no_pnglib__
+#ifdef __MINT__
+#undef _GNU_SOURCE
+#endif
 #include <stdlib.h>
 #include <string.h>
 #include <assert.h>