[pycrypto] setup.py fails to find gmp on Debian/Ubuntu

[Legrandin]

> Debian and Ubuntu recently started to implement Multiarch [1]. Because of this,
> libgmp.so has been moved from /usr/lib/ to /usr/lib/<host-triplet>/ and setup.py
> cannot find it anymore.
>
> I was able to circumvent this problem by asking gcc for its library search path
> (see the attached patch). I don't know if that is the best way to fix this issue
> but I would be glad if you'd consider the patch or fix the issue differently.

> [1] https://wiki.ubuntu.com/MultiarchSpec

Hi Sebastian, all,

I think the problem is more deep rooted.

Searching the file system for the gmp library file is not very
reliable, in that one has to make a lot of assumptions about how the
linker works.
Moreover, we don't know if the file we find is actually in the right
binary format or not.

I suggest a different approach. We get rid of the searching logic
altogether, and we simply try to compile a dummy C file that requires
libgmp. If it compiles, we know that the library is somehow reachable,
but we don't need to investigate where it actually is.

That is what has_function() in distutils is supposed to do, but I find
it very limited: the sought function can only be one that does not
take any argument, whereas ALL functions in libgmp I know of take at
least one.

See attached patch.

Mind that I have only tried it with Python 2.7 on Linux.
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