Use C99 to compile ISL

ISL with small integer optimization requires C99 to compile. gcc < 5.0
still uses C89 as default, so we need to enable the options to compile
in C99 mode.

This patch is preparing the actual activation of small integer
optimization.

Differential version: http://reviews.llvm.org/D10610

Reviewers: grosser
llvm-svn: 240322