config: Conditionalize -finline-functions on compiler support

config: Conditionalize -finline-functions on compiler support

Certain clang compiler versions do not support -finline-functions, so only apply
this compiler option conditionally if supported.

Warnings with Apple LLVM version 8.0.0 (clang-800.0.42.1), based on upstream clang 3.9.0:

  clang: warning: optimization flag '-finline-functions' is not supported
  clang: warning: argument unused during compilation: '-finline-functions'

Fixes: 7dd9a7f ("Add -finline-functions to standard build flags, so gcc will
                 consider all functions as candidates for inlining.")

Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>