Add vlibc scaffold, build system, and documentation

Establish the repository layout per Vox guidelines (layouts/C.md,
building/C.md): Autotools build (GCC-only, C23), five compatibility
profiles (--enable-onlyposix/--enable-muslmimic/--enable-muslext/
--enable-spoof, default vlibc), alongside/overwrite install methods,
vlibc-gcc/vlibc-clang drivers, static-linking requirement (except
spoof), benchmark harness, and tooling (.clang-format/.clang-tidy/
.clangd + compile_commands.json).

.gitignore

@@ -52,3 +52,54 @@ Module.symvers
 Mkfile.old
 dkms.conf
 
+# Autotools backup files (autoreconf/autoheader).
+*~
+*.orig
+*.rej
+
+# ---> vlibc / Autotools
+# Generated build system. `configure` and `autogen.sh` are tracked (layouts/C.md);
+# the Makefiles and other artifacts produced from configure.ac / Makefile.am are not.
+Makefile
+Makefile.in
+.dirstamp
+/aclocal.m4
+/autom4te.cache/
+/config.h
+/config.h.in
+/config.log
+/config.status
+/config.guess
+/config.sub
+/install-sh
+/missing
+/depcomp
+/ltmain.sh
+/libtool
+/libtool.m4
+/ltoptions.m4
+/ltsugar.m4
+/ltversion.m4
+/lt~obsolete.m4
+/stamp-h1
+/build-aux/
+/m4/
+
+# Libtool build directories and artifacts.
+.deps/
+.libs/
+*.lo
+
+# Build outputs (bin/{release,debug}); keep the directory placeholders.
+/bin/release/*
+!/bin/release/.gitkeep
+/bin/debug/*
+!/bin/debug/.gitkeep
+
+# Generated compiler drivers (from tools/*.in); the templates are tracked.
+/tools/vlibc-gcc
+/tools/vlibc-clang
+
+# Built benchmark harnesses (benchmarks/bench_* are generated from bench_*.c).
+/benchmarks/bench_vlibc
+