PROTON-412: Fix the include and lib directories in libqpid-proton.pc

If the paths aren't passed in from the command line as absolute then
prepends them with CMAKE_INSTALL_PREFIX before generated the package
config file.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1517673 13f79535-47bb-0310-9956-ffa450edef68