meson: fix logic for generating .pc files with old glvnd

meson: fix logic for generating .pc files with old glvnd

We want to generate PC files for non-glvnd builds and for builds with
old glvnd, but the current logic doesn't do that, it builds them
unconditionally, and for GLES it builds the shared libraries, which is
also not what we want. This does not generate .pc files for gles1 or
gles2. Which it we weren't doing before either, making this not a
regression but a return to status-quo.o

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1838
Fixes: 93df862b6affb6b8507e40601212a58012bfa873
       ("meson: re-add incorrect pkg-config files with GLVND for backward compatibility")
Reviewed-by: Matt Turner <mattst88@gmail.com>