TOOLS: add macos-sdk-version.py script

Building for macos requires us to check the macos sdk path as well as
the sdk version that is on the system. To do this, let's steal the logic
that's in the compiler_swift.py check from the waf build. This returns a
comma-delinated string. The first entry is the absolute path to the sdk.
The second entry is the detected macos sdk version.

TOOLS/macos-sdk-version.py

@@ -0,0 +1,68 @@
+#!/usr/bin/env python3
+
+# Logic copied from compiler_swift.py in the waf build. This checks for the sdk
+# path, the sdk version, and the sdk build version. The sdk path is returned
+# along with what is selected as the sdk version.
+
+import re
+import os
+import string
+import sys
+from distutils.version import StrictVersion
+from shutil import which
+from subprocess import check_output
+
+def find_macos_sdk():
+    sdk = os.environ.get('MACOS_SDK', '')
+    sdk_version = os.environ.get('MACOS_SDK_VERSION', '0.0')
+    xcrun = which('xcrun')
+
+    if not xcrun:
+        return sdk,sdk_version
+
+    if not sdk:
+        sdk = check_output([xcrun, '--sdk', 'macosx', '--show-sdk-path'],
+                            encoding="UTF-8")
+    
+    # find macOS SDK paths and version
+    if sdk_version == '0.0':
+        # show-sdk-build-version: is not available on older command line tools, but returns a build version (eg 17A360)
+        # show-sdk-version: is always available, but on older dev tools it's only the major version
+        sdk_build_version = check_output([xcrun, '--sdk', 'macosx',
+                                          '--show-sdk-build-version'], encoding="UTF-8")
+
+        sdk_version = check_output([xcrun, '--sdk', 'macosx', '--show-sdk-version'],
+                                    encoding="UTF-8")
+
+    if sdk:
+        build_version = '10.10.0'
+
+    # convert build version to a version string
+    # first 2 two digits are the major version, starting with 15 which is 10.11 (offset of 4)
+    # 1 char is the minor version, A => 0, B => 1 and ongoing
+    # last digits are bugfix version, which are not relevant for us
+    # eg 16E185 => 10.12.4, 17A360 => 10.13, 18B71 => 10.14.1
+    if sdk_build_version and isinstance(sdk_build_version, str):
+        verRe = re.compile("(\d+)(\D+)(\d+)")
+        version_parts = verRe.search(sdk_build_version)
+        major = int(version_parts.group(1)) - 4
+        minor = string.ascii_lowercase.index(version_parts.group(2).lower())
+        build_version = '10.' + str(major) + '.' + str(minor)
+        # from 20 onwards macOS 11.0 starts
+        if int(version_parts.group(1)) >= 20:
+            build_version = '11.' + str(minor)
+
+    if not isinstance(sdk_version, str):
+        sdk_version = '10.10.0'
+
+    # pick the higher version, always pick sdk over build if newer
+    if StrictVersion(build_version) > StrictVersion(sdk_version):
+        return sdk,build_version
+    else:
+        return sdk,sdk_version
+
+    return sdk,sdk_version
+
+if __name__ == "__main__":
+    sdk_info = find_macos_sdk()
+    sys.stdout.write(','.join(sdk_info))