build(linux): introduce overlay makefile infrastructure

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- Delete versions/26.00/…/Bundles/Nsis7z/makefile.gcc (superseded by the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.

versions/26.00/CPP/7zip/Bundles/Nsis7z/Nsis7z.def

@@ -1,6 +0,0 @@
-LIBRARY nsis7z
-EXPORTS
-  Extract
-  ExtractWithDetails
-  ExtractWithCallback
-  ExtractWithFileCallback

versions/26.00/CPP/7zip/Bundles/Nsis7z/makefile.gcc

@@ -1,97 +0,0 @@
-PROG = nsis7z
-DEF_FILE = Nsis7z.def
-
-include ../Format7zF/Arc_gcc.mak
-
-ifdef IS_MINGW
-LOCAL_FLAGS_SYS = -DZ7_DEVICE_FILE
-SYS_OBJS = \
-  $O/DLL.o \
-  $O/DllSecur.o \
-  $O/resource.o \
-
-else
-SYS_OBJS = \
-  $O/MyWindows.o \
-
-endif
-
-LOCAL_FLAGS += \
-  $(LOCAL_FLAGS_SYS) \
-  $(LOCAL_FLAGS_EXTRA) \
-
-NSIS_UI_OBJS = \
-  $O/NsisMain.o \
-  $O/NsisMainAr.o \
-  $O/NsisBreak.o \
-  $O/NsisExtractCallbackConsole.o \
-  $O/NsisUserInputUtils2.o \
-  $O/ConsoleOpenCallbackConsole.o \
-  $O/ConsolePercentPrinter.o \
-  $O/ConsoleClose.o \
-  $O/ConsoleUserInputUtils.o \
-  $O/Extract.o \
-  $O/HashCalc.o \
-  $O/OpenArchive.o \
-  $O/ArchiveExtractCallback.o \
-  $O/ArchiveOpenCallback.o \
-  $O/DefaultName.o \
-  $O/EnumDirItems.o \
-  $O/ExtractingFilePath.o \
-  $O/LoadCodecs.o \
-  $O/SetProperties.o \
-  $O/FileStreams.o \
-  $O/FileSystem.o \
-  $O/ErrorMsg.o \
-  $O/FileLink.o \
-  $O/FilePathAutoRename.o \
-  $O/SortUtils.o \
-  $O/PropIDUtils.o \
-
-CURRENT_OBJS = \
-  $O/StdInStream.o \
-  $O/StdOutStream.o \
-  $O/StdAfx2.o \
-  $O/pluginapi.o \
-  $O/nsis7z.o \
-
-OBJS = \
-  $(ARC_OBJS) \
-  $(SYS_OBJS) \
-  $(NSIS_UI_OBJS) \
-  $(CURRENT_OBJS) \
-
-include ../../7zip_gcc.mak
-
-$O/NsisMain.o: ../../UI/NSIS/Main.cpp
-	$(CXX) $(CXXFLAGS) $<
-
-$O/NsisMainAr.o: ../../UI/NSIS/MainAr.cpp
-	$(CXX) $(CXXFLAGS) $<
-
-$O/NsisBreak.o: ../../UI/NSIS/NSISBreak.cpp
-	$(CXX) $(CXXFLAGS) $<
-
-$O/NsisExtractCallbackConsole.o: ../../UI/NSIS/ExtractCallbackConsole.cpp
-	$(CXX) $(CXXFLAGS) $<
-
-$O/NsisUserInputUtils2.o: ../../UI/NSIS/UserInputUtils2.cpp
-	$(CXX) $(CXXFLAGS) $<
-
-$O/ConsoleOpenCallbackConsole.o: ../../UI/Console/OpenCallbackConsole.cpp
-	$(CXX) $(CXXFLAGS) $<
-
-$O/ConsolePercentPrinter.o: ../../UI/Console/PercentPrinter.cpp
-	$(CXX) $(CXXFLAGS) $<
-
-$O/ConsoleUserInputUtils.o: ../../UI/Console/UserInputUtils.cpp
-	$(CXX) $(CXXFLAGS) $<
-
-$O/StdAfx2.o: StdAfx2.cpp
-	$(CXX) $(CXXFLAGS) $<
-
-$O/pluginapi.o: pluginapi.cpp
-	$(CXX) $(CXXFLAGS) $<
-
-$O/nsis7z.o: nsis7z.cpp
-	$(CXX) $(CXXFLAGS) $<