flac: fix benchmark linking issue with older glibc versions

flac: fix benchmark linking issue with older glibc versions

Fixes:
http://autobuild.buildroot.net/results/b64/b643824d797050d7c001a3b72598158d8f1c6128/
http://autobuild.buildroot.net/results/a2f/a2f2d7b4e9e7b57bc8691e3ec29ecf818be3f79b/

The benchmark tool uses clock_gettime which is available in librt instead of
libc in older glibc versions.  The build system correctly links with -lrt on
Linux systems, but misdetects non-x86 linux systems as !linux, causing build
failures.

Add a patch to relax the linux OS detection to also work on non-x86 systems.

Patch submitted upstream:
http://lists.xiph.org/pipermail/flac-dev/2017-January/006103.html

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>