Fix how CMake flags get propagated into pycompadre setup

[Paul Kuberry]

When running:

>> CMAKE_CONFIG_FILE=cmake_opts.txt python setup.py install

or

>> CMAKE_CONFIG_FILE=cmake_opts.txt python setup.py bdist_wheel

it is clear that the configuration options in cmake_opts.txt will be respected.

When installing from a wheel, there is no problem because these arguments are built into the wheel. (verify)

When installing from source using pypi, pip install pycompadre, the source will have a cmake_opts.txt in the root of the source code, but nothing will indicate to the setup call that it should use this .txt file, so it will be ignored.

Need to make it so that if CMAKE_CONFIG_FILE is not specified, then it will default to the cmake_opts.txt shipped with the source .tar.gz file.