Fix issues with newer clang

- Clang now complains about declarations after statements in C code,
  even when the standard is set to something higher than C99. Disable
  that warning (but only do this for C source, because GCC doesn't like
  that warning applied to C++ sources).

- As of Clang 14, the default for fp-contract is "on", even when
  optimizations are disabled. This was leading to many test failures.
  https://releases.llvm.org/14.0.0/tools/clang/docs/ReleaseNotes.html#floating-point-support-in-clang

Change-Id: I8cafd5bbde60d629ee5a26e69fd8e7c212296b10
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/583741
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed