Pass through all arguments to CC and CXX to configure

Previously, if one attempted to pass arguments to the compiler by
setting the `CC` or `CXX` environment variables, the additional arguments
would be ignored and not passed on to the configure scripts for cfitsio
and healpix_cxx. For instance, if one ran setup.py as follows:

    $ env CC="gcc -foo -bar" CXX="g++ -foo -bar" python setup.py build...

then the `-foo` and `-bar` arguments would not be passed on when
building cfitsio or healpix_cxx.

With this patch, the `CC` and `CXX` environment variables are propagated to
the cfitsio and healpix_cxx builds including any additional command line
arguments.

This partially addresses #136 and #151, in which it is necessary to pass
some additional arguments to the C++ compiler to build on Anaconda for
Mac OS.