meson: Fix GL and EGL pkg-config files with glvnd

meson: Fix GL and EGL pkg-config files with glvnd

Currently meson will generate a pkg-config that links to EGL_mesa (or
GLX_mesa), but this isn't correct, it should always link to EGL or GL.
Probably the "right" solution is to have glvnd itself provide the pkg
config files for GL and EGL, but that also means that glvnd needs to
provide many of the header files, which makes it a more involved job.

Fixes: a47c525f3281a27 ("meson: build glx")
Fixes: 035ec7a2bb2d5e4 ("meson: Add support for EGL glvnd")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>