[zstd] Move zstd.module to external/

It felt clunky to have zstd.module in src/libraries/zstd by itself, and
doesn't make much sense in src/libraries as it's an external library
anyway.

Now the ./configure script will pick up .module files in the top-level
external directory as well.

src/libraries/zstd/zstd.module

@@ -1,33 +0,0 @@
-# vim: ft=python
-
-zstd = module("zstd",
-    root = "${source_root}/external/zstd",
-    kind = "lib",
-    deps = [ "libc" ],
-    output = "libzstd.a",
-    sources = [
-        "decompress/zstd_decompress.c",
-        "decompress/zstd_ddict.c",
-        "decompress/huf_decompress.c",
-        "decompress/zstd_decompress_block.c",
-        "common/threading.c",
-        "common/fse_decompress.c",
-        "common/debug.c",
-        "common/xxhash.c",
-        "common/pool.c",
-        "common/error_private.c",
-        "common/entropy_common.c",
-        "common/zstd_common.c",
-    ],
-    public_headers = [
-        "zdict.h",
-        "zstd.h",
-        "zstd_errors.h",
-    ])
-
-zstd.variables['ccflags'] = [
-    "${ccflags}", 
-    "-DXXH_NAMESPACE=ZSTD_",
-    "-DDEBUGLEVEL=0",
-    "-DZSTD_DISABLE_ASM",
-]