Conda weave path

This small commit adds `$prefix/include` to the list of included header files in weave. When running in a normal Python environment, this should do nothing (e.g. prefix is `/usr` or `/usr/local`, but `/usr/include` etc. are standard include dirs anyway), but when run under anaconda this will add an anaconda-specific include directory. Before you had to do this manually in the configuration, otherwise weave compilation failed due to mismatches of the math libraries.

If this doesn't screw things up for Windows, please merge.