title: add flags to cffi compile process
body: <p>I use cffi module to wrap a simple c code. the problem is, I need to add a flag to make it compile (std=c99).
  currently I have something like that:</p>
  
  <pre><code>from cffi import FFI
  ffibuilder = FFI()
  with open("test.c", 'r') as f:
      ffibuilder.set_source("mymodule", f.read()) 
  with open("test.h", 'r') as f:
      ffibuilder.cdef(f.read())
  if __name__ == "__main__":
      ffibuilder.compile(verbose=True)
  </code></pre>
  
  <p>The problem is, that cffi calls gcc by itself and I want to add std=c99 to the flags it calls gcc with.
  Any parameter I am missing?</p>
  
  <p>(Note: I can change gcc command itself or run the gcc command cffi uses myself, I am wondering if I am missing some correct way to do it)</p>