CMake: Fix MinGW missing winsock ws2_32 lib linking

[Challanger524]

PR for: chriskohlhoff/asio#1590

MSYS2 MinGW not linking ws2_32: undefined reference to `__imp_WSAStartup'

This issue is old as world.

MSYS2/mingw64/GCC: undefined reference to `__imp_WSAStartup'

[build] C:\windows\system32\cmd.exe /C "cd . && C:\msys64\mingw64\bin\g++.exe -g  CMakeFiles/test-beast.dir/src/main.cpp.obj -o C:\*\dev\test-boost-beast\test-beast.exe -Wl,--out-implib,libtest-beast.dll.a -Wl,--major-image-version,0,--minor-image-version,0  dep/boost/libs/context/libboost_context-gcc14-mt-d-x64-1_88.a  dep/boost/libs/date_time/libboost_date_time-gcc14-mt-d-x64-1_88.a  dep/boost/libs/container/libboost_container-gcc14-mt-d-x64-1_88.a  -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 && cd ."
[build] C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/14.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: CMakeFiles/test-beast.dir/src/main.cpp.obj: in function `boost::asio::detail::winsock_init_base::startup(boost::asio::detail::winsock_init_base::data&, unsigned char, unsigned char)':
[build] C:/*/dev/boost/libs/asio/include/boost/asio/detail/impl/winsock_init.ipp:39:(.text$_ZN5boost4asio6detail17winsock_init_base7startupERNS2_4dataEhh[_ZN5boost4asio6detail17winsock_init_base7startupERNS2_4dataEhh]+0x36): undefined reference to `__imp_WSAStartup'

MSYS2/clang64/Clang: ld.lld: error: undefined symbol: __declspec(dllimport) WSAStartup

[build] C:\windows\system32\cmd.exe /C "cd . && C:\msys64\clang64\bin\clang++.exe -g  CMakeFiles/test-beast.dir/src/main.cpp.obj -o C:\*\dev\test-boost-beast\test-beast.exe -Wl,--out-implib,libtest-beast.dll.a -Wl,--major-image-version,0,--minor-image-version,0  dep/boost/libs/context/libboost_context-clang19-mt-d-x64-1_88.a  dep/boost/libs/date_time/libboost_date_time-clang19-mt-d-x64-1_88.a  dep/boost/libs/container/libboost_container-clang19-mt-d-x64-1_88.a  -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 && cd ."
[build] ld.lld: error: undefined symbol: __declspec(dllimport) WSAStartup
[build] >>> referenced by C:/*/dev/boost/libs/asio/include/boost/asio/detail/impl/winsock_init.ipp:39
[build] >>>               CMakeFiles/test-beast.dir/src/main.cpp.obj:(boost::asio::detail::winsock_init_base::startup(boost::asio::detail::winsock_init_base::data&, unsigned char, unsigned char))

Solution / workaround

Simply link library ws2_32 to your or asio project.

if (MINGW)
  target_link_libraries(${PROJECT_NAME} PRIVATE ws2_32) # mswsock wsock32
endif()

or

if (MINGW)
  target_link_libraries(boost_asio INTERFACE ws2_32) # mswsock wsock32
endif()

Credits

try to build cmake+mingw+win10/64bit problem : boostorg/beast#672 (comment)
Installing boost libraries for GCC (MinGW) on Windows: https://gist.github.com/Shivam60/4a3a4923616ed317a9a094a8173c3391

Asio solution

As I understand - Asio author uses C++ source files to handle library includes (I have no words): https://github.com/chriskohlhoff/asio/blob/62481a25be6cf78cbe714419a4462fd89bd84ab9/asio/include/asio/detail/socket_types.hpp#L47-L53