Add INTERNAL_FUSE variable to control linking against
bundled libfuse vs external/system libfuse.
If external libfuse is requested,
`pkg-config` is used to obtain CFLAGS/LIBS/LDFLAGS and
to check that the provided external libfuse is >= 2.9.7.
This might hopefully get mergerfs closer to inclusion into distros.
Link: https://github.com/trapexit/mergerfs/issues/431
Link: https://bugs.gentoo.org/587956#c6