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>

groundwater/uClibc,ffainelli/uClibc,hjl-tools/uClibc,wbx-github/uclibc-ng,foss-for-synopsys-dwc-arc-processors/uClibc,ddcc/klee-uclibc-0.9.33.2,skristiansson/uClibc-or1k,groundwater/uClibc,brgl/uclibc-ng,ddcc/klee-uclibc-0.9.33.2,hwoarang/uClibc,hjl-tools/uClibc,ddcc/klee-uclibc-0.9.33.2,foss-xtensa/uClibc,ddcc/klee-uclibc-0.9.33.2,foss-for-synopsys-dwc-arc-processors/uClibc,kraj/uclibc-ng,majek/uclibc-vx32,majek/uclibc-vx32,hwoarang/uClibc,waweber/uclibc-clang,hjl-tools/uClibc,czankel/xtensa-uclibc,groundwater/uClibc,foss-xtensa/uClibc,czankel/xtensa-uclibc,ffainelli/uClibc,groundwater/uClibc,foss-for-synopsys-dwc-arc-processors/uClibc,waweber/uclibc-clang,waweber/uclibc-clang,ffainelli/uClibc,brgl/uclibc-ng,hwoarang/uClibc,czankel/xtensa-uclibc,groundwater/uClibc,wbx-github/uclibc-ng,mephi42/uClibc,kraj/uclibc-ng,skristiansson/uClibc-or1k,czankel/xtensa-uclibc,ffainelli/uClibc,wbx-github/uclibc-ng,foss-xtensa/uClibc,hjl-tools/uClibc,kraj/uClibc,majek/uclibc-vx32,kraj/uclibc-ng,kraj/uClibc,majek/uclibc-vx32,skristiansson/uClibc-or1k,kraj/uclibc-ng,skristiansson/uClibc-or1k,kraj/uClibc,brgl/uclibc-ng,waweber/uclibc-clang,ffainelli/uClibc,kraj/uClibc,hwoarang/uClibc,mephi42/uClibc,wbx-github/uclibc-ng,foss-for-synopsys-dwc-arc-processors/uClibc,foss-xtensa/uClibc,mephi42/uClibc,brgl/uclibc-ng,hjl-tools/uClibc,mephi42/uClibc