Add MinGW (GCC) build support on Windows

[DUDUACM]

• src/MCFloat.h: #undef FP_SNAN/FP_QNAN under MinGW only - mingw-w64's math.h defines them as macros (MSVC does not), breaking the enum
• mdns_cpp/src/mdns.cpp: use in4addr_any only when compiling with MSVC; the constant is not declared in mingw-w64 headers, fall back to INADDR_ANY which is equivalent and portable
• CMakeLists.txt:
  • call project() early so the compiler is identified before the MSVC runtime flag setup, and keep those flags inherited by mdns_cpp
  • apply WIN_RUNTIME_LIB (-MD/-MT) flags only for MSVC; GCC rejects -MDd (invalid in Debug builds)
  • link mswsock when not using MSVC: boost::asio needs AcceptEx/GetAcceptExSockaddrs, which MSVC auto-links via #pragma comment(lib) in the asio headers but MinGW does not
  • fix Windows UNITTEST executable name: get_target_property on the unset OUTPUT_NAME returned NOTFOUND, producing an executable named "OUTPUT_NAME_OLD-NOTFOUND-MD.exe"

Verified with xerces-c 3.3.0 + boost 1.86.0 + MVR_XChange + unit tests: MSVC Release/Debug and MinGW GCC 13.1 Release/Debug all build and pass all 27 unit test suites. No behavior change for MSVC builds.

[welcome]

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