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.
Merge pull request #152 from lpsinger/pass-thru-all-cc-cxx-args

Pass through all arguments to CC and CXX to configure