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.

CMakeLists.txt

@@ -428,7 +428,7 @@ endforeach()
 configure_file(${CMAKE_CURRENT_SOURCE_DIR}/builds/cmake/platform.hpp.in ${CMAKE_CURRENT_BINARY_DIR}/platform.hpp)
 list(APPEND sources ${CMAKE_CURRENT_BINARY_DIR}/platform.hpp)
 
-configure_file(${CMAKE_CURRENT_SOURCE_DIR}/src/libzmq.pc.cmake.in ${CMAKE_CURRENT_BINARY_DIR}/libzmq.pc)
+configure_file(${CMAKE_CURRENT_SOURCE_DIR}/src/libzmq.pc.cmake.in ${CMAKE_CURRENT_BINARY_DIR}/libzmq.pc @ONLY)
 set(zmq-pkgconfig ${CMAKE_CURRENT_BINARY_DIR}/libzmq.pc)
 
 if(NOT ZMQ_BUILD_FRAMEWORK)

src/libzmq.pc.cmake.in

 prefix=@CMAKE_INSTALL_PREFIX@
-exec_prefix=@CMAKE_INSTALL_PREFIX@
-libdir=@CMAKE_INSTALL_PREFIX@/lib
-includedir=@CMAKE_INSTALL_PREFIX@/include
+exec_prefix=${prefix}
+libdir=${prefix}/lib
+includedir=${prefix}/include
 
 Name: libzmq
 Description: 0MQ c++ library
 Version: @ZMQ_VERSION_MAJOR@.@ZMQ_VERSION_MINOR@.@ZMQ_VERSION_PATCH@
 Libs: -L${libdir} -lzmq
-Cflags: -I@CMAKE_INSTALL_PREFIX@/include
+Cflags: -I${includedir}