Fix compilation of object file by changing .c suffixes to .cpp suffixes so make actually runs our target to build the object file rather than its own default target.  Also introduce EXTRA_CXXFLAGS for project specific settings that should *always* be included in a build but allows CXXFLAGS to be overridden at build time e.g. make CXXFLAGS="my preferred compile options"
Merge pull request #8 from grahamwhiteuk/master

Compilation now acts as expected, also introduces EXTRA_CXXFLAGS.