ci(windows): stop udp-discovery-cpp being compiled as C++98

c++0x_warning.h:32: error: This file requires compiler and library support
    for the ISO C++ 2011 standard

udp-discovery-cpp asks CMake for C++98, so the sub-build is compiled with
-std=gnu++98, and its own sources then include headers that need C++11. The
UCRT64 toolchain refuses; the older GCC on the Ubuntu runner lets it through,
which is why only Windows stops here.

Two reasons it does not happen in the super-project: it carries
udp-discovery-cpp as a submodule pinned at f3a3103, so the add_subdirectory
branch is taken -- and the FetchContent branch used by a standalone build asks
for GIT_TAG "origin/master", the moving tip of a third party project. That is
worth reporting on its own: this build follows whatever that repository does
today.

CMAKE_CXX98_*_COMPILE_OPTION is what CMake emits when a target asks for C++98.
Remapping it to gnu++17 leaves every other target untouched, and keeps this out
of CMakeLists.txt.

.github/workflows/windows-cmake.yml

@@ -77,6 +77,8 @@ jobs:
           -DRS_SQLCIPHER=ON -DRS_BITDHT=ON
           -DRS_BRODCAST_DISCOVERY=ON -DRS_MINIUPNPC=ON
           -DRS_FORUM_DEEP_INDEX=OFF
+          -DCMAKE_CXX98_STANDARD_COMPILE_OPTION=-std=gnu++17
+          -DCMAKE_CXX98_EXTENSION_COMPILE_OPTION=-std=gnu++17
           -DCMAKE_C_FLAGS="-include string.h -include stdint.h"
           -DCMAKE_CXX_FLAGS="-include cstring -include cstdint -Wno-template-body"