glapi: fix _glapi_get_proc_address() for mangled function names

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

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94994
Cc: "11.1 11.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mapi/glapi/glapi_getproc.c

@@ -62,12 +62,7 @@ get_static_proc( const char * n )
    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 @@ _glapi_get_proc_address(const char *funcName)
 
    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)