Fix CMake-generated `libzmq.pc` file

This change mirrors the Autotools-based build system behavior for
cross-compiling for Windows with static linking.

See https://github.com/zeromq/libzmq/pull/4706.


diff --git a/CMakeLists.txt b/CMakeLists.txt
index 03462271..0315e606 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -546,12 +546,18 @@ if(ZMQ_HAVE_WINDOWS)
   # Cannot use check_library_exists because the symbol is always declared as char(*)(void)
   set(CMAKE_REQUIRED_LIBRARIES "ws2_32.lib")
   check_cxx_symbol_exists(WSAStartup "winsock2.h" HAVE_WS2_32)
+  if(HAVE_WS2_32)
+    set(pkg_config_libs_private "${pkg_config_libs_private} -lws2_32")
+  endif()
 
   set(CMAKE_REQUIRED_LIBRARIES "rpcrt4.lib")
   check_cxx_symbol_exists(UuidCreateSequential "rpc.h" HAVE_RPCRT4)
 
   set(CMAKE_REQUIRED_LIBRARIES "iphlpapi.lib")
   check_cxx_symbol_exists(GetAdaptersAddresses "winsock2.h;iphlpapi.h" HAVE_IPHLAPI)
+  if(HAVE_IPHLAPI)
+    set(pkg_config_libs_private "${pkg_config_libs_private} -liphlpapi")
+  endif()
   check_cxx_symbol_exists(if_nametoindex "iphlpapi.h" HAVE_IF_NAMETOINDEX)
 
   set(CMAKE_REQUIRED_LIBRARIES "")