Issue239 clang nopie

[mikebentley15]

Fixes #239

Description:

• Add an extra conditional check for Clang within the generated Makefile
• For Clang, use -nopie instead of -no-pie when linking
• CC changed to CXX to reflect it as a C++ compiler instead of a C compiler
• Remove "MPI is enabled" message from the Makefile

Documentation:
No documentation was updated since this was already supposed to be supported.

I suppose we should document that we support Clang 3.4+ and GCC 4.8+

Tests:

• Updated the MPI test removing the section checking for the "MPI is enabled" message
• Added tests for Clang 3.4 and GCC 4 where the compilers are faked and checked against known allowed flags.

[mikebentley15]

The documentation has been updated to reflect the minimum versions of each compiler.

[mikebentley15]

Also fixes #191