Add custom variable to jack pkgconfig to distinguish implementations

jack.pc.in:
Add the custom `jack_implementation=jack1` pkgconfig variable to the
generated jack.pc file to be able to distinguish jack implementations.
As jack implementations exist with jack1, jack2 and pipewire-jack, it is
not (easily) possible to distinguish them by looking at the version
alone (particularly the case with jack2 vs. pipewire-jack, as they share
the same headers).

jack.pc.in

@@ -2,6 +2,7 @@ prefix=@prefix@
 exec_prefix=@exec_prefix@
 libdir=@libdir@
 includedir=@includedir@
+jack_implementation=jack1
 
 Name: jack
 Description: the Jack Audio Connection Kit: a low-latency synchronous callback-based media server