Do not include RPATHs in the install tree.

Normally CMake uses the build tree for the RPATH when building executables etc on systems that use RPATH. When the software is installed the executables etc are relinked by CMake to have the install RPATH. If this variable is set to true then the software is always installed without RPATH, even if RPATH is enabled when building. This can be useful for example to allow running tests from the build directory with RPATH enabled before the installation step. To omit RPATH in both the build and install steps, use CMAKE_SKIP_RPATH instead.