build: add version.py for generating version.h

version.h is essential for building, and its generation was done by a
shell script. Strictly speaking, python should in general be more
portable (windows), and would be better for the upcoming meson build to
simply just execute a python script. version.py has some small
differences with version.sh which shouldn't matter but they are noted
below.

- version.sh accepted several arguments that seemed useless (like
  --cwd). These were removed from version.py.
- version.py takes either no arguments (prints the version) or it takes
  exactly one argument specifying the complete path of where the header
  should be generated.
- version.sh attempted to read a file named "snapshot_version". The
  comments noted that this was for "daily tarball snapshots". Such a
  file does not exist in the source tree, and it's not really clear that
  anyone actually uses this. This logic was removed from version.py.
- version.py reads the SOURCE_DATE_EPOCH environment variable. Some
  distros use this for reproducible builds. Technically you could also
  just disable the build date but this is only a couple of extra lines
  and maybe it's prettier than UNKNOWN.
- version.py also doesn't attempt to display timezone information in the
  build date. It only shows UTC time.

version.py

@@ -0,0 +1,45 @@
+#!/usr/bin/env python3
+
+import os
+import sys
+import time
+
+from datetime import datetime,timezone
+from shutil import which
+from subprocess import check_output
+
+srcdir = os.path.dirname(os.path.abspath(sys.argv[0]))
+git_dir = os.path.join(srcdir, ".git")
+git = which('git')
+
+if git and os.path.exists(git_dir):
+    version = check_output([git, "-C", srcdir, "describe", "--always", "--tags",
+                            "--dirty"], encoding="UTF-8")
+    version = version[1:].strip()
+else:
+    version_path = os.path.join(srcdir, "VERSION")
+    with open(version_path, "r") as f:
+        version = f.readline().strip()
+
+if len(sys.argv) < 2:
+    print(version)
+    sys.exit()
+
+date = datetime.utcfromtimestamp(int(os.environ.get('SOURCE_DATE_EPOCH', time.time())))
+if date == "":
+    date = datetime.now(timezone.utc).astimezone()
+date_str = date.strftime("%a %b %d %I:%M:%S %Y")
+
+NEW_REVISION = "#define VERSION \"" + version + "\"\n"
+OLD_REVISION = ""
+BUILDDATE = "#define BUILDDATE \"" + date_str + "\"\n"
+MPVCOPYRIGHT = "#define MPVCOPYRIGHT \"Copyright \u00A9 2000-2021 mpv/MPlayer/mplayer2 projects\"" + "\n"
+
+if os.path.isfile(sys.argv[1]):
+    with open(sys.argv[1], "r") as f:
+        OLD_REVISION = f.readline()
+
+if NEW_REVISION != OLD_REVISION:
+    with open(sys.argv[1], "w", encoding="utf-8") as f:
+        f.writelines([NEW_REVISION, BUILDDATE, MPVCOPYRIGHT])
+