Fix building with libpng-1.5

https://bugs.gentoo.org/show_bug.cgi?id=378353

Patch written by Samuli Suominen <ssuominen@gentoo.org>
--- a/GL/glpng/glpng.c
+++ b/GL/glpng/glpng.c
@@ -285,7 +285,7 @@
     endinfo = png_create_info_struct(png);
 
     // DH: added following lines
-    if (setjmp(png->jmpbuf))
+    if (setjmp(png_jmpbuf(png)))
     {
         png_destroy_read_struct(&png, &info, &endinfo);
         return 0;
@@ -390,7 +390,7 @@
     endinfo = png_create_info_struct(png);
 
     // DH: added following lines
-    if (setjmp(png->jmpbuf))
+    if (setjmp(png_jmpbuf(png)))
     {
         png_destroy_read_struct(&png, &info, &endinfo);
         return 0;
@@ -569,7 +569,7 @@
 #define ALPHA *q
 
                 switch (trans) {
-                    case PNG_CALLBACK:
+                    case PNG_CALLBACKT:
                         FORSTART
                             ALPHA = AlphaCallback((unsigned char) r, (unsigned char) g, (unsigned char) b);
                         FOREND
--- a/GL/glpng/glpng.h
+++ b/GL/glpng/glpng.h
@@ -57,7 +57,7 @@
 #define PNG_SIMPLEMIPMAP PNG_SIMPLEMIPMAPS
 
     /* Transparency parameters */
-#define PNG_CALLBACK  -3 /* Call the callback function to generate alpha   */
+#define PNG_CALLBACKT  -3 /* Call the callback function to generate alpha   */
 #define PNG_ALPHA     -2 /* Use alpha channel in PNG file, if there is one */
 #define PNG_SOLID     -1 /* No transparency                                */
 #define PNG_STENCIL    0 /* Sets alpha to 0 for r=g=b=0, 1 otherwise       */