Move the check for whether to dlsym or GPA on linux to the common C code.

This is going to change for macos and win32, and this will be easier
than trying to spread that logic through the python code and into the
generated code.

src/dispatch_common.c

@@ -338,6 +338,16 @@ epoxy_gles2_dlsym(const char *name)
     return do_dlsym(&api.gles1_handle, "libGLESv2.so.2", name, true);
 }
 
+void *
+epoxy_get_core_proc_address(const char *name, int core_version)
+{
+    if (core_version <= 12) {
+        return epoxy_gl_dlsym(name);
+    } else {
+        return epoxy_get_proc_address(name);
+    }
+}
+
 void *
 epoxy_get_proc_address(const char *name)
 {

src/dispatch_common.h

@@ -55,6 +55,7 @@ void *epoxy_gl_dlsym(const char *name);
 void *epoxy_gles1_dlsym(const char *name);
 void *epoxy_gles2_dlsym(const char *name);
 void *epoxy_get_proc_address(const char *name);
+void *epoxy_get_core_proc_address(const char *name, int core_version);
 
 int epoxy_conservative_gl_version(void);
 bool epoxy_conservative_has_gl_extension(const char *name);

src/gen_dispatch.py

@@ -282,14 +282,8 @@ class Generator(object):
                 human_name = 'Desktop OpenGL {0}'.format(feature.get('number'))
                 condition = 'epoxy_is_desktop_gl()'
 
-                # Everything in GL 1.2 is guaranteed to be present as
-                # public symbols in the Linux libGL ABI.  Everything
-                # else is supposed to not be present, so you have to
-                # glXGetProcAddress() it.
-                if version <= 12:
-                    loader = 'epoxy_gl_dlsym({0})'
-                else:
-                    loader = 'epoxy_get_proc_address({0})'
+                loader = 'epoxy_get_core_proc_address({0}, {1})'.format('{0}', version)
+                if version >= 11:
                     condition += ' && epoxy_conservative_gl_version() >= {0}'.format(version)
             elif api == 'gles2':
                 human_name = 'OpenGL ES {0}'.format(feature.get('number'))