Fix compiler flags for GCC
Fix compiler flags for GCC

This brings IREE closer to beeing build with GCC, see https://github.com/google/iree/issues/6#issuecomment-544168619. `-Wno-strict-prototypes` is supported by GCC in general, but only for C/Objective-C. Therefore, moving it to the CLANG flags.

Closes #159

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/iree/pull/159 from marbre:gcc_flags e01bb061ed3d955f98c1c9a979b4b7b4dd8aa327
PiperOrigin-RevId: 284778723

iree-org/iree,iree-org/iree,google/iree,iree-org/iree,iree-org/iree,iree-org/iree,iree-org/iree,google/iree,google/iree,google/iree,google/iree,iree-org/iree,google/iree,google/iree