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).

src/vlibc.c

@@ -0,0 +1,16 @@
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+#include <vlibc.h>
+
+/*
+ * The version string is defined here, in a single translation unit, rather
+ * than in the header, so that only the library owns the object. Consumers use
+ * the VLIBC_VERSION_STRING macro for compile-time access.
+ */
+const char *
+vlibc_version(void)
+{
+    return VLIBC_VERSION_STRING;
+}