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).

autogen.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+# vlibc — Autotools bootstrap (layouts/C.md).
+#
+# Regenerates the build system (configure, Makefile.in, ...) from the
+# *.ac / *.am sources and then runs configure, passing through any
+# arguments. The generated `configure` script is tracked; the Makefiles
+# it produces are not (see .gitignore).
+
+set -e
+
+srcdir=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+cd "$srcdir"
+
+autoreconf -i -f
+
+./configure "$@"