c++ on Cygwin is missing the 'const char *' redefinition of strchr, and...

c++ on Cygwin is missing the 'const char *' redefinition of strchr, and complains angrily that this test does not get the signature right, so tell it to be more permissive.

tests/run/cpdef_extern_func.pyx

 # cython: c_string_type=str
 # cython: c_string_encoding=ascii
+# distutils: extra_compile_args=-fpermissive
 
 __doc__ = """
 >>> sqrt(1)