mesa: make use of HAVE_FUNC_ATTRIBUTE_ALIAS macro

We must make sure that xserver has an equivalent one-line
change to its configure.ac as the glx/glapi headers get copied over.

Then again, xserver does _not_ seem to set HAVE_ALIAS to begin with so
one might want to look into that first.

Cc: Adam Jackson <ajax@redhat.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

scons/gallium.py

@@ -323,10 +323,6 @@ def generate(env):
                 'GLX_DIRECT_RENDERING',
                 'GLX_INDIRECT_RENDERING',
             ]
-        if env['platform'] in ('linux', 'freebsd'):
-            cppdefines += ['HAVE_ALIAS']
-        else:
-            cppdefines += ['GLX_ALIAS_UNSUPPORTED']
 
         if env['platform'] in ('linux', 'darwin'):
             cppdefines += ['HAVE_XLOCALE_H']