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).

benchmarks/Makefile.am

@@ -0,0 +1,27 @@
+# vlibc — benchmark harnesses (musts/BENCHMARKING.md).
+
+AM_CPPFLAGS = -I$(top_srcdir)/include
+AM_CFLAGS = @VLIBC_CFLAGS@
+
+# Built only on demand (via `make bench`), so `make all` does not need the
+# library to be built first.
+EXTRA_PROGRAMS = bench_vlibc
+bench_vlibc_SOURCES = bench_vlibc.c
+
+# By default benchmarks link against vlibc itself. --with-libc=glibc links the
+# same harness against the system glibc; --with-libc=musl builds it with
+# musl-gcc for the musl reference.
+if BENCH_LINK_VLIBC
+bench_vlibc_LDADD = ../libvlibc.la
+else
+bench_vlibc_LDADD =
+endif
+
+if BENCH_LINK_MUSL
+CC = $(MUSL_CC)
+endif
+
+bench: bench_vlibc
+	./bench_vlibc
+
+.PHONY: bench