Replaces the per-version in-tree build files with a shared overlay that
keeps all project-owned build infrastructure outside the vendor source trees,
making it safe to use vendor directories as pristine git submodules.
Changes:
- Add tools/linux/overlay/makefile.gcc: parameterised makefile that drives
MinGW cross-compilation; VENDOR_7ZIP, DEF_FILE and NSIS_DIR are all
overridable from the command line so no files are written into the vendor
tree at build time.
- Add tools/linux/overlay/include/: case-sensitivity shim headers
(Windows.h, CommCtrl.h, MAPI.h, NTSecAPI.h, Psapi.h, ShlObj.h) that
redirect to the lowercase MinGW system headers; also adds
z7_idecl_noexcept_strip.h which resolves the noexcept-specifier mismatch
in 7-zip 25.01 NSIS UI code.
- Move Nsis7z.def from versions/26.00/…/Bundles/Nsis7z/ to overlay/ (git
rename; content unchanged).
- Delete versions/26.00/…/Bundles/Nsis7z/makefile.gcc (superseded by the
overlay).
- Rewrite tools/linux/build_plugin_linux.py to drive the overlay makefile,
support multiple 7-zip versions (25.01, 26.00, 26.01, zstd) with a
VERSION_LAYOUT table, and add NSIS_DIR auto-detection for bundle-based
builds.
- Add _linux_build/ to .gitignore.
