Build failure on MinGW from #5066

[kmilos]

@nrusch The 3.1.12.0 change in #5066 broke the MinGW build w/:

FAILED: [code=1] bin/iconvert.exe 
  C:\Windows\system32\cmd.exe /C "cd . && D:\M\msys64\ucrt64\bin\g++.exe -march=nocona -msahf -mtune=generic -O2 -pipe -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector-strong -Wp,-D__USE_MINGW_ANSI_STDIO=1 -O3 -DNDEBUG /MANIFEST:NO src/iconvert/CMakeFiles/iconvert.dir/iconvert.cpp.obj src/iconvert/CMakeFiles/iconvert.dir/__/windows/oiio_exe.rc.obj -o bin\iconvert.exe -Wl,--out-implib,lib\libiconvert.dll.a -Wl,--major-image-version,0,--minor-image-version,0  lib/libOpenImageIO.dll.a  lib/libOpenImageIO_Util.dll.a  -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 && cd ."
  D:/M/msys64/ucrt64/lib/gcc/x86_64-w64-mingw32/15.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find /MANIFEST:NO: No such file or directory
  collect2.exe: error: ld returned 1 exit status

The addition of that manifest switch only works for MSVC, but adding the .rc should still work w/ MinGW.

[kmilos]

FWIW, the alternative to direct switch control could be something like
https://github.com/libjxl/libjxl/blob/37bbfd09b4fd42554940ac2c0b72a1121cfa6d8a/tools/CMakeLists.txt#L332-L342

[lgritz]

Can somebody who understands Windows and MinGW (I do not) please take the initiative to do any or all of the following?

• Turn this issue's advice into a PR that fixes it?
• Modify our CI to test MinGW so we can keep it working? (Is there even a way to test MinGW in CI with GHA?)
• Modify our INSTALL.md to precisely define our level of support on MinGW?

[officialasishkumar]

I've put up a PR for the first point — #5129 wraps the /MANIFEST:NO switch in an if(MSVC) guard so MinGW links cleanly while still embedding the manifest resource on all Windows builds.

Happy to look into the CI and INSTALL.md pieces too if nobody else picks those up.