configure: Prefix libc-related variables with "libc_"

Simplifies host/target libc detection splitting.
Merge commit 'd64341e498fa46c6cf4c23d408177d7489559654'

* commit 'd64341e498fa46c6cf4c23d408177d7489559654':
  configure: Prefix libc-related variables with "libc_"

Conflicts:
	libavdevice/Makefile
	libavformat/Makefile

Merged-by: Michael Niedermayer <michaelni@gmx.at>
cmdutils & opencl: add -opencl_bench option to test and show available OpenCL devices

Reviewed-by: Wei Gao <highgod0401@gmail.com>
Reviewed-by: Stefano Sabatini <stefasab@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>