Add configure-time test for latest ISL

Query the isl_config.h macros recently added to ISL. One of it looks for
the ffs (find first set), whose functionality is available in Visual
Studio with _BitScanForward. Also add isl_ffs.c to the source files
which contains the implementation of ffs using _BitScanForward.

Reviewers: grosser
llvm-svn: 242770