This PR implements a few changes in the way suite-sparse is found on installation. Previously the include and library paths were hardcocded. Now they can be controlled through environment variables:

If you suite-sparse library is installed in a non-standard place and you get errors when installing with pip you can use the environment
variables:

• SUITESPARSE_INCLUDE_DIR
• SUITESPARSE_LIBRARY_DIR

at runtime so the compiler can find them. For example, lets say your suite-sparse installation is in /opt/local then you can run

SUITESPARSE_INCLUDE_DIR=/opt/local/include SUITESPARSE_LIBRARY_DIR=/opt/local/lib pip install scikit-sparse

This should fix #67