PkgConfigDependency: Cache the output of pkg-config
PkgConfigDependency: Cache the output of pkg-config

This halves the configure time in gst-build, the aggregate of all
GStreamer repositories.

We can't do this to Popen_safe because we can't be sure that other
programs have no side-effects and will always return the same output
for the same arguments and environment.

mesonbuild/meson,jeandet/meson,jpakkane/meson,jeandet/meson,jeandet/meson,thiblahute/meson,jeandet/meson,jeandet/meson,jpakkane/meson,pexip/meson,QuLogic/meson,QuLogic/meson,jpakkane/meson,jeandet/meson,pexip/meson,jeandet/meson,becm/meson,pexip/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,becm/meson,pexip/meson,becm/meson,MathieuDuponchelle/meson,thiblahute/meson,mesonbuild/meson,thiblahute/meson,thiblahute/meson,becm/meson,QuLogic/meson,pexip/meson,jpakkane/meson,MathieuDuponchelle/meson,jeandet/meson,thiblahute/meson,MathieuDuponchelle/meson,QuLogic/meson,jpakkane/meson,becm/meson,pexip/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,MathieuDuponchelle/meson,QuLogic/meson,thiblahute/meson,MathieuDuponchelle/meson,jpakkane/meson,mesonbuild/meson,thiblahute/meson,jpakkane/meson,MathieuDuponchelle/meson,becm/meson,mesonbuild/meson,MathieuDuponchelle/meson,QuLogic/meson,MathieuDuponchelle/meson,jpakkane/meson,pexip/meson,pexip/meson,mesonbuild/meson,jeandet/meson,mesonbuild/meson,mesonbuild/meson,MathieuDuponchelle/meson,thiblahute/meson,becm/meson,mesonbuild/meson,QuLogic/meson,becm/meson,QuLogic/meson,thiblahute/meson,becm/meson,becm/meson