Run code coverage on your local machine

This change introduces a new make target called "coverage" it will
compile your code using special GCC flags, run the tests, and then
generate a nice HTML output.

This new make target will only be available if you build using GCC in
Debug mode.

To build the code coverage and open it in your browser do this:

    mkdir debug
    cd debug
    cmake -DCMAKE_BUILD_TYPE=Debug -DENABLE_COVERAGE=ON ..
    make -j20
    make coverage
    xdg-open coverage/index.html
Moved local code coverage code separate script
Include coverage script if ENABLE_COVERAGE is set

Rather than including the LibarchiveCodeCoverage.cmake file and checking
there if ENABLE_COVERAGE is set, we only include the file if this option
is set in the first place.
Merge branch 'local_coverage' of github.com:kwk/libarchive into kwk-local_coverage

Conflicts:
	CMakeLists.txt