has_function() tries to link an example program using the function
to see if it is available, but with clang on 64bit Windows this
example always already failed at the compile step:
error: cast from pointer to smaller type 'long' loses information
long b = (long) a;
This is due to long!=pointer with LLP64
Change from "long" to "long long" which is min 64bit and should always
fit a pointer. While "long long" is strictly a C99 feature every
non super ancient compiler should still support it.
c4fa0fac3d