avoid exporting absolute path

[dirk-thomas]

Otherwise the install prefix is not relocatable.

Related to ros2/ros2#915.

[jacobperron]

lgtm

[lopsided98]

While CMAKE_INSTALL_FULL_LIBDIR is guaranteed to be a absolute path, CMAKE_INSTALL_LIBDIR is not guaranteed to be a path relative to CMAKE_INSTALL_PREFIX, and may be an absolute path as well.

Something like this: https://github.com/jtojnar/cmake-snips#concatenating-paths-when-building-pkg-config-files is needed here (although obviously applied to the CMake config file rather than the pkg-config file). This would allow the package to be relocatable if CMAKE_INSTALL_LIBDIR is a relative path, but still work if it is absolute.

[lopsided98]

See #66 for a fix.