build-sys: remove PKG_PREREQ
build-sys: remove PKG_PREREQ

I added it to make sure AC_SUBST happened, but apparently AC_SUBST
happens with very old pkg-config versions too, no need to check
pkg-config pkg.m4 version.

Signed-off-by: Marc-André Lureau <a6f2b960a4aca61381fc650a67901b2ac8e83e6d@redhat.com>

fgouget/spice-gtk,flexVDI/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk