Filter platform-specific predefined macros in exdef

[grian32]

Problem

exdef uses clang -dM -E, which includes compiler/platform predefined macros alongside the header-defined constants. The code already skips linux and unix, but macOS/Windows-specific predefined macros can still leak into generated .gl3 output.

Tasks

• Identify common macOS predefined macros that should be excluded.
• Identify common Windows/MSVC/clang-cl-style predefined macros that should be excluded.
• Replace the hardcoded linux/unix check with a clearer platform/predefined macro filter.
• Keep user header defines with similar-looking names from being accidentally filtered if possible.
• Add exdef tests/fixtures for platform predefined macro filtering.

Refs: cli/exdef.go.