[Polly] Add -std=c99 flag only to C source files

Summary: Adding the flag to C++ source files emits a warning, hence we set the compile flag depending on the file's language.

Reviewers: grosser

Subscribers: Meinersbur, pollydev, llvm-commits

Projects: #polly

Differential Revision: http://reviews.llvm.org/D10809

llvm-svn: 240986