Improve MinGW compatibility

[Ashutosh0x]

This Pull Request addresses several build and link issues encountered when compiling MuJoCo with the MinGW-w64 toolchain.

Changes

• Fix stat redefinition: Added #ifndef stat around the #define stat _stat in user_vfs.cc. MinGW-w64 often defines stat as a macro (e.g., _stat64), causing a conflict with MuJoCo's manual mapping.
• Support thread-safe localtime: Changed the check in engine_util_errmem.c from #elif _MSC_VER to #elif defined(_WIN32). MinGW-w64 provides the same localtime_s function as MSVC, but it was previously hidden.
• Fix stack size flag: Updated MujocoLinkOptions.cmake to use -Wl,--stack,size instead of -Wl,/STACK:size when using the GNU compiler on Windows. GNU ld does not recognize the MSVC-style /STACK flag.

These changes combined with PR #3086 (printf format errors) should allow MuJoCo to build out-of-the-box with modern MinGW-w64 environments.

Fixes #3037, #3038, #3040

[mmossg]

Why is this needed when you already have #3083 and #3084?

[Ashutosh0x]

Why is this needed when you already have #3083 and #3084?

Thanks for checking!

PR #3083 and #3084 addressed specific Windows/MinGW issues individually, but during testing I found additional build and linking problems that still prevented a clean out-of-the-box MinGW-w64 build.

This PR bundles the remaining fixes (stat macro conflict, localtime_s detection, and GNU ld stack flag) so MuJoCo can compile successfully in a modern MinGW environment end-to-end

[Ashutosh0x]

I've removed the redundant linker flag changes, as they are already covered in #3083. This PR now solely focuses on the stats macro and thread-safe localtime fixes.

[mmossg]

#3084 has the stats and local time fixes.