Fix and simplify rpath

- GMXLIB was removed by 0b2344 but forgotten to change for rpath.
- CMAKE_SKIP_BUILD_RPATH and CMAKE_BUILD_WITH_INSTALL_RPATH
  default to false
- No need to relink (CMAKE_BUILD_WITH_INSTALL_RPATH=false) if
  installing in the default lib directory. Saves the relinking
  time and makes rpath work correctly if build folder is moved.
- The extra escaping isn't needed anymore. Tested with 2.8.8,
  2.8.10.2 and 2.8.12.2

Change-Id: I09cf0f9a31c2c891d2aec6ab52dd9cd1d089cb97