cmake: Refer to prefix variable in generated libzmq.pc

This change:
1. Makes the `libzmq.pc` files generated by Autotools and CMake more
aligned.
2. Allows the `prefix` variable to be redefined if the package is
relocated.

CMakeLists.txt

@@ -1256,9 +1256,9 @@ configure_file(${CMAKE_CURRENT_SOURCE_DIR}/builds/cmake/platform.hpp.in ${CMAKE_
 list(APPEND sources ${CMAKE_CURRENT_BINARY_DIR}/platform.hpp)
 
 set(prefix ${CMAKE_INSTALL_PREFIX})
-set(exec_prefix ${prefix})
-set(libdir ${prefix}/${CMAKE_INSTALL_LIBDIR})
-set(includedir ${prefix}/${CMAKE_INSTALL_INCLUDEDIR})
+set(exec_prefix "\${prefix}")
+set(libdir "\${prefix}/${CMAKE_INSTALL_LIBDIR}")
+set(includedir "\${prefix}/${CMAKE_INSTALL_INCLUDEDIR}")
 set(VERSION ${ZMQ_VERSION_MAJOR}.${ZMQ_VERSION_MINOR}.${ZMQ_VERSION_PATCH})
 configure_file(${CMAKE_CURRENT_SOURCE_DIR}/src/libzmq.pc.in ${CMAKE_CURRENT_BINARY_DIR}/libzmq.pc @ONLY)
 set(zmq-pkgconfig ${CMAKE_CURRENT_BINARY_DIR}/libzmq.pc)