only link against geos C API on non-windows platforms

Currently trying to build basemap on Debian Linux using the system wide global GEOS installation fails (installed via aptitude, see https://packages.debian.org/search?keywords=libgeos&searchon=names&suite=stable&section=all).

This is due to basemap trying to link against the C++ library and looking for a file `/usr/lib/libgeos.so` while GEOS only provides `/usr/lib/libgeos-3.3.3.so`. I reported this circumstance to the Debian package maintainer and he replied that it is bad practice to link against the C++ library (see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=741242). Following his reply I removed `'geos'` from the library list and basemap builds OK and also the normal plotting seems to work fine.

For Debian Linux this PR makes basemap install like a charm simply using `pip install basemap` (with the respective options to allow files not directly hosted on pypi), removing the need to download the source code and manually compiling the GEOS library included in the source.

I am not sure if this creates problems in other parts of basemap, though...?