librt: refactor source selection
librt: refactor source selection

Should make adding more knob control simpler as people only need
to modify one variable now (librt_filter_SRC).

Signed-off-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@gentoo.org>

joel-porquet/tsar-uclibc,joel-porquet/tsar-uclibc,joel-porquet/tsar-uclibc,joel-porquet/tsar-uclibc