CMake: fail fast on Windows (PSLP is POSIX-only)

[LucasBoTang]

As reported in issue #67, the presolve module PSLP depends on POSIX APIs and headers and is not compatible with MSVC on Windows. This currently results in confusing build failures during compilation for Windows users.

Thus, this PR adds a configure-time guard to explicitly fail on Windows platforms. Rather than allowing CMake to generate a build system that fails during compilation, this change makes the limitation explicit and fails fast with a clear error message and suggested workarounds. Failing early at configuration time provides a significantly better user experience and avoids unnecessary debugging effort.

What this change does

• Adds a WIN32 check in the root CMakeLists.txt.
• Emits a FATAL_ERROR during configuration with:
  • the reason for the failure (PSLP is POSIX-only),
  • and clear workarounds (Linux / WSL2 / older known-working releases).