meson: Improve RPATH handling

meson: Improve RPATH handling

Right now we're unconditionally adding RPATH information to the
installed binaries and libraries, but that's not always desired.

autotools seem to be smart enough to only include that information
when targeting a non-standard prefix, so most distro packages
don't actually contain it; moreover, both Debian and Fedora have
wiki pages encouraging packagers to avoid setting RPATH:

  https://wiki.debian.org/RpathIssue
  https://fedoraproject.org/wiki/RPath_Packaging_Draft

Implement RPATH logic that Does The Right Thing™ in the most
common cases, while still offering users the ability to override
the default behavior if they have specific needs.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>