Problem: cmake: pkgconfig always installed in lib.

Solution: Use cmake's libdir variable to ensure that the pkgconfig file
ends up in the correct lib/lib64 directory. This matches the autotools
behaviour.

CMakeLists.txt

@@ -900,7 +900,7 @@ configure_file (${CMAKE_CURRENT_SOURCE_DIR}/src/libzmq.pc.cmake.in ${CMAKE_CURRE
 set (zmq-pkgconfig ${CMAKE_CURRENT_BINARY_DIR}/libzmq.pc)
 
 if (NOT ZMQ_BUILD_FRAMEWORK)
-  install (FILES ${CMAKE_CURRENT_BINARY_DIR}/libzmq.pc DESTINATION lib/pkgconfig)
+  install (FILES ${CMAKE_CURRENT_BINARY_DIR}/libzmq.pc DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig)
 endif ()
 
 if (MSVC)