rtmidi doesn't find jack in a non standard path. (Or the default pipewire path)

[simotek]

For some background the pipewire implementation of libjack, commonly installs libjack.so into /usr/lib64/pipewire-0.3/jack/ but also ships pkgconfig files so pkgconf jack --exists returns the correct information as below

pkgconf jack --cflags -> -D_REENTRANT
pkgconf jack --libs -> -L/usr/lib64/pipewire-0.3/jack -ljack

This should provide all the information needed to compile the library correctly. For what its worth everything compiles fine with symlink's to /usr/lib64/. My knowledge of autotools isn't good so I can't provide a patch to easily fix the issue myself but i'm happy to test.

• Expected behavior and actual behavior.
  rtmidi should compile against libjack in non standard locations if the system is setup correctly.

• Steps to reproduce the problem.
  Try and compile rtmidi on a openSUSE, Fedora or Debian with pipewire-jack installed instead of jack.

• Specifications like the version of the project, operating system, or hardware.
  Personally i'm using openSUSE Tumbleweed with pipewire-jack 0.3.77
  https://packages.debian.org/sid/amd64/pipewire-jack/filelist seems to indicate it would also be an issue on debian based systems.
  does not detect jack if provided by pipewire-jack-audio-connection-kit-devel supercollider/supercollider#5678 probably points to it also being an issue on fedora systems as well.