Support specifying the installation RPATH

Hi Simon,

This patch adds the capability to specify the installation RPATH when building kcov.

The use case for this is where kcov is being built using a newer g++ version (due to C++11 usage) and then deployed to another machine that doesn't have the newer libstdc++ library available in the expected location (without having to set LD_LIBRARY_PATH).

Example usage:
cd kcov
mkdir build
cd build
cmake -DSPECIFY_RPATH=TRUE -DCMAKE_INSTALL_PREFIX=/local/testsuite
make
make install

The installed version of the kcov executables have the RPATH set to include $CMAKE_INSTALL_PREFIX/lib.