configure.ac: error out if python/mako is not found when required
configure.ac: error out if python/mako is not found when required

In case of using a distribution tarball (or a dirty git tree) one can
have the generated sources locally. Make configure.ac error out
otherwise, to alert that about the unmet requirement(s) of python/mako.

v2: Check only for a single file for each dependency.

Suggested-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>
Signed-off-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>
Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler