In the dispatch table, all functions are stored without the "m" prefix.
Modify code so that OSMesaGetProcAddress works both with gl and mgl prefixes.
Similar to https://lists.freedesktop.org/archives/mesa-dev/2015-September/095251.html

--- mesa-11.1.2/src/mapi/glapi/glapi_getproc.c.bak	2016-01-18 08:39:26.000000000 +0100
+++ mesa-11.1.2/src/mapi/glapi/glapi_getproc.c	2016-04-18 11:35:19.000000000 +0200
@@ -62,12 +62,7 @@
    GLuint i;
    for (i = 0; static_functions[i].Name_offset >= 0; i++) {
       const char *testName = gl_string_table + static_functions[i].Name_offset;
-#ifdef MANGLE
-      /* skip the prefix on the name */
-      if (strcmp(testName, n + 1) == 0)
-#else
       if (strcmp(testName, n) == 0)
-#endif
       {
 	 return &static_functions[i];
       }
@@ -516,15 +511,14 @@
 
    init_glapi_relocs_once();
 
-#ifdef MANGLE
-   /* skip the prefix on the name */
-   if (funcName[1] != 'g' || funcName[2] != 'l')
-      return NULL;
-#else
-   if (funcName[0] != 'g' || funcName[1] != 'l')
-      return NULL;
+#ifdef USE_MGL_NAMESPACE
+   if (funcName && funcName[0] == 'm')
+      funcName++;
 #endif
 
+  if (!funcName || funcName[0] != 'g' || funcName[1] != 'l')
+      return NULL;
+
    /* search extension functions first */
    func = get_extension_proc_address(funcName);
    if (func)