fix compatibility with >=libpng-1.4
https://bugs.gentoo.org/show_bug.cgi?id=491610

diff -Nur darkplaces.old/image_png.c darkplaces/image_png.c
--- darkplaces.old/image_png.c	2009-09-15 06:08:02.000000000 +0300
+++ darkplaces/image_png.c	2010-01-28 01:23:24.000000000 +0200
@@ -36,7 +36,7 @@
 static void*			(*qpng_create_info_struct)	(void*);
 static void				(*qpng_read_info)			(void*, void*);
 static void				(*qpng_set_expand)			(void*);
-static void				(*qpng_set_gray_1_2_4_to_8)	(void*);
+static void				(*qpng_set_expand_gray_1_2_4_to_8)	(void*);
 static void				(*qpng_set_palette_to_rgb)	(void*);
 static void				(*qpng_set_tRNS_to_alpha)	(void*);
 static void				(*qpng_set_gray_to_rgb)		(void*);
@@ -61,7 +61,7 @@
 	{"png_create_info_struct",	(void **) &qpng_create_info_struct},
 	{"png_read_info",			(void **) &qpng_read_info},
 	{"png_set_expand",			(void **) &qpng_set_expand},
-	{"png_set_gray_1_2_4_to_8",	(void **) &qpng_set_gray_1_2_4_to_8},
+	{"png_set_expand_gray_1_2_4_to_8",	(void **) &qpng_set_expand_gray_1_2_4_to_8},
 	{"png_set_palette_to_rgb",	(void **) &qpng_set_palette_to_rgb},
 	{"png_set_tRNS_to_alpha",	(void **) &qpng_set_tRNS_to_alpha},
 	{"png_set_gray_to_rgb",		(void **) &qpng_set_gray_to_rgb},
@@ -110,7 +110,7 @@
 #elif defined(MACOSX)
 		"libpng12.0.dylib",
 #else
-		"libpng12.so.0",
+		"libpng14.so.0",
 		"libpng.so", // FreeBSD
 #endif
 		NULL
@@ -311,7 +311,7 @@
 	{
 		qpng_set_gray_to_rgb(png);
 		if (my_png.BitDepth < 8)
-			qpng_set_gray_1_2_4_to_8(png);
+			qpng_set_expand_gray_1_2_4_to_8(png);
 	}
 
 	if (qpng_get_valid(png, pnginfo, PNG_INFO_tRNS))