Cannot be used as a meson subproject (wrap)

[dcbaker]

meson.source_root() will always be the toplevel source root, so if libepoxy is used as a wrap the generation of epoxy_headers will fail since it will look in the for include/epoxy in the parent project, not in epoxy itself.

[ebassi]

Indeed, we use meson.source_root() twice:

• once to find the gen_dispatch.py script, to work around a Meson bug that was already fixed a couple of releases ago
• once to construct a full path to the headers in order to generate appropriate dependencies, as we refer to those files from different directories

Both should go away regardless.

[ebassi]

@dcbaker I've opened a PR that should fix the issue; testing is appreciated.