jack.pc.in:
Add the custom `jack_implementation=jack2` 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).

Signed-off-by: falkTX <falktx@falktx.com>

git-svn-id: http://subversion.jackaudio.org/jack/jack2/trunk/jackmp@3075 0c269be4-1314-0410-8aa9-9f06e86f4224

git-svn-id: http://subversion.jackaudio.org/jack/jack2/trunk/jackmp@1990 0c269be4-1314-0410-8aa9-9f06e86f4224