fix CMakeLists.txt to find numpy and f2py

[bnavigator]

Some distributions install architecture specific python packages into directory structures like /usr/lib64/ instead of /usr/lib/. This patch allows CMake to find numpy and f2py in these directories.

CMake probably provides more sophisticated routines to find the correct python packages, but this patch is the closest to the existing routine I could come up with.

[roryyorke]

Thanks for the PR! Could you take a look at #67, which I'm planning to merge soon? @jakevdp found we needed to use the numpy "root" (see #67 diff) instead of get_python_lib for cases when numpy is installed in a "non-standard place"---would you mind trying that to find fortranobject.c?

[bnavigator]

Yes that also works, too. See the build output at
https://build.opensuse.org/package/live_build_log/home:bnavigator/python-slycot/openSUSE_Tumbleweed/x86_64
(Although I just realized from that output that there are problems with the prefix)

I did not realize that $PYTHON_SITE is indeed only used by CMake to look for the numpy path. Disregard this PR then and merge #67.