Spruce up pkg-config files for libzfs/libzfs_core

Several of the listed library dependencies are not relevant on FreeBSD.
Have ./configure save libraries that are found via pkg-config as
${LIB}_PC and use the configured automake variables instead of hard
coded names so we only get what was actually needed.

While here, update the URL to point at the OpenZFS Github repo.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10869