Disable JSCoverage and it's building in OS X.

The development toolchain that comes with Mavericks
does not play well with compiling JSCoverage -- which is
unmaintained anyway.

Till we move to a better test coverage tool, we should
disable it therefore people on OS X can compile WebODF.
Merge pull request #191 from adityab/mavericks-disable-jscoverage

Disable JSCoverage and it's building in OS X.