build: improve cross-compilation and toolchain detection
- Export CC, CXX, FC, and binutils variables (AR, AS, LD, etc.) for subprojects
- Fix FC_VERSION detection to use -dumpversion instead of preprocessor macro
- Add --with-pic for GMP on non-Windows platforms for position-independent code
- Fix GMP build on Windows/AArch64 by disabling -DPIC which causes :got: relocation errors on COFF
- Skip -mconsole flag on Clang for Windows builds (not supported)
- Fix OpenBLAS incoming-stack-boundary flag to only apply on i686, not all non-x86_64
- Fix OpenSSL target selection to handle AArch64 Windows (mingw64)
- Fix OpenSSL clean/distclean targets with incorrect path prefix
- Fix Zstd Windows detection to use OS instead of TARGET_SYSTEM
- Fix FC_VERSION in refresh_checksums.mk to use short version format
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
