Lior Goldberg authored Dec 25, 2015 and Brad King committed Jan 15, 2016

Some compilers may add their own RPATH entries when invoking the linker.
For example, a GCC installation may contain the following definition in
the specs file:

  *link_libgcc:
  %D -rpath <<some specific rpath in which libstdc++.so can be found>>

In this case binaries may contain RPATH entries that CMake did not add.
When we update the RPATH on installation we must preserve these entries
even if CMake thinks the INSTALL_RPATH value should be empty.

Fix this by always using file(RPATH_CHANGE) and teach it to behave as
file(RPATH_REMOVE) if the actual RPATH in the file is empty after
replacing the build-tree RPATH with the install-tree RPATH.  This will
preserve any compiler-added RPATH value instead of removing it.