New subproject win-nasm to provide nasm on Windows

This is needed for building openh264 as a subproject. Currently it's
downloaded unconditionally since the download is very small.

meson.build

@@ -57,6 +57,7 @@ os.symlink(os.path.join('@1@', 'subprojects', '@0@'),
 
 if build_system == 'windows'
   subproject('win-flex-bison-binaries')
+  subproject('win-nasm')
 endif
 
 subproject('orc', required: get_option('orc'))

subprojects/win-nasm/.gitignore

@@ -0,0 +1,2 @@
+nasm-*/
+nasm-*.zip

subprojects/win-nasm/download-binary.py

@@ -0,0 +1,51 @@
+#!/usr/bin/env python3
+
+import os
+import sys
+import ssl
+import zipfile
+import hashlib
+import urllib.request
+
+# Disable certificate checking because it always fails on Windows
+# We verify the checksum anyway.
+ctx = ssl.create_default_context()
+ctx.check_hostname = False
+ctx.verify_mode = ssl.CERT_NONE
+
+base_url = 'https://www.nasm.us/pub/nasm/releasebuilds/{0}/{1}/nasm-{0}-{1}.zip'
+arch = 'win64' if sys.argv[2] == 'x86_64' else 'win32'
+url = base_url.format(sys.argv[1], arch)
+zip_sha256 = sys.argv[3]
+source_dir = os.path.join(os.environ['MESON_SOURCE_ROOT'], os.environ['MESON_SUBDIR'])
+dest = os.path.basename(url)
+dest_path = os.path.join(source_dir, dest)
+
+def get_sha256(zipf):
+    hasher = hashlib.sha256()
+    with open(zipf, 'rb') as f:
+        hasher.update(f.read())
+    return hasher.hexdigest()
+
+if os.path.isfile(dest_path):
+    found_sha256 = get_sha256(dest_path)
+    if found_sha256 == zip_sha256:
+        print('{} already downloaded'.format(dest))
+        sys.exit(0)
+    else:
+        print('{} checksum mismatch, redownloading'.format(dest))
+
+print('Downloading {} to {}'.format(url, dest))
+with open(dest_path, 'wb') as d:
+    f = urllib.request.urlopen(url, context=ctx)
+    d.write(f.read())
+
+found_sha256 = get_sha256(dest_path)
+if found_sha256 != zip_sha256:
+    print('SHA256 of downloaded file {} was {} instead of {}'
+          ''.format(dest, found_sha256, zip_sha256))
+    sys.exit(1)
+
+print('Extracting {}'.format(dest))
+zf = zipfile.ZipFile(dest_path, "r")
+zf.extractall(path=source_dir)

subprojects/win-nasm/meson.build

@@ -0,0 +1,20 @@
+project('win-nasm', version : '2.14.02')
+
+py3 = import('python3').find_python()
+
+message('Downloading and extracting nasm binaries for Windows...')
+
+arch = host_machine.cpu_family()
+if arch == 'x86_64'
+  zip_hash = '18918ac906e29417b936466e7a2517068206c8db8c04b9762a5befa18bfea5f0'
+else
+  zip_hash = '250f9b5eeb2111e8c7b494a977490985b8604fe7518a6f5041cde37cc727a067'
+endif
+
+ret = run_command(py3, files('download-binary.py'), meson.project_version(), arch, zip_hash)
+if ret.returncode() != 0
+  message(ret.stdout())
+  error(ret.stderr())
+endif
+
+meson.override_find_program('nasm', find_program(join_paths('nasm-@0@'.format(meson.project_version()), 'nasm')))