Builds of OSMesa compile with a clean checkout

This CL adds a gyp target and a few scripts which generate mesa's created
sources. Please note that this adds dependencies on a few utilities:

- ylwrap, currently hardcoded to /usr/share/automake1.11/ylwrap
- flex
- bison
- python2 (though this can hopefully be replaced with Chromium's
      supported version of Python

Also note that one of the build steps creates an executable compiler,
which is then executed to generate a source file. This change adds
a relevant 'executable' target to mesa's gyp file, and makes its
dependent target rely on that executable.