fix libzmq.pc generation under cmake

The ${libdir} was getting replaced/removed by configure_file() making pkg-config give bad flags: -L -lzmq
My fix was to add @ONLY to configure_file() so ${} style pkg-config substitutions are left alone.
In addition, I put the other typical ${} substitutions back into the libzmq.pc, since its now safe.
Merge pull request #884 from guruofquality/cmake_fixes

fix libzmq.pc generation under cmake