Currently all the build systems but Meson generate the header in
src/mapi/glapi. Meson cannot do that since:
 - it does not allow user control over the location of output files
 - moving the generation rule(s) causes explosion due to the unusual
structure of glapi and friends
 - copying the file into the correct location is a non-trivial task

To workaround the above deficiency in the least invasive way, let's
adjust the #include directive and add a few -I flags to the autotools
build.

Note: both builddir and srcdir, should be used. Otherwise building from
a release tarball fails badly.

Cc: Dylan Baker <dylanx.c.baker@intel.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Tested-by: Mark Janes <mark.a.janes@intel.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>