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).

Makefile.am

@@ -0,0 +1,56 @@
+# vlibc — top-level build (see building/C.md and layouts/C.md).
+
+SUBDIRS = benchmarks
+
+AM_CPPFLAGS = -I$(top_srcdir)/include
+AM_CFLAGS = @VLIBC_CFLAGS@
+
+# ---- Library -------------------------------------------------------------
+vlibc_include_HEADERS = include/vlibc.h
+vlibc_lib_LTLIBRARIES = libvlibc.la
+
+libvlibc_la_SOURCES = src/vlibc.c
+libvlibc_la_LDFLAGS = -version-info 0:0:0 -no-undefined
+
+# Install location depends on the install method (see configure.ac).
+if INSTALL_OVERWRITE
+vlibc_includedir = $(includedir)
+vlibc_libdir = $(libdir)
+else
+vlibc_includedir = $(prefix)/lib/vlibc/include
+vlibc_libdir = $(prefix)/lib/vlibc/lib
+endif
+
+# ---- Compiler drivers ----------------------------------------------------
+bin_SCRIPTS = tools/vlibc-gcc tools/vlibc-clang
+
+# ---- Targets -------------------------------------------------------------
+# layouts/C.md requires `make debug`, `make release`, `make bench` and
+# `make clean`; build outputs land in bin/{release,debug}.
+DEBUG_CFLAGS = -O0 -g3 -DDEBUG
+RELEASE_CFLAGS = -O3 -DNDEBUG
+
+debug:
+	$(MAKE) clean
+	$(MAKE) all AM_CFLAGS="$(AM_CFLAGS) $(DEBUG_CFLAGS)"
+	$(MKDIR_P) bin/debug
+	-cp -P .libs/libvlibc.so* bin/debug/
+	-cp .libs/libvlibc.a bin/debug/
+
+release:
+	$(MAKE) clean
+	$(MAKE) all AM_CFLAGS="$(AM_CFLAGS) $(RELEASE_CFLAGS)"
+	$(MKDIR_P) bin/release
+	-cp -P .libs/libvlibc.so* bin/release/
+	-cp .libs/libvlibc.a bin/release/
+
+bench: all
+	$(MAKE) -C benchmarks bench
+
+# Regenerate compile_commands.json (requires `bear`). Captures both the
+# library sources and the on-demand benchmark harness.
+compile-commands: clean
+	bear -- $(MAKE) all
+	bear --append -- $(MAKE) -C benchmarks bench_vlibc
+
+.PHONY: debug release bench compile-commands