Update CMAKE_OSX_DEPLOYMENT_TARGET from 10.11 to 10.14 (#924)

Apparently 10.11 is no longer good enough to run the latest version of
binaryen.  Specifically since binaryen switched to using std::variant it
now fails to build with this set to 10.11.

This is also the version used on the emscripten-releases CI which builds
the emsdk binaries:
https://chromium.googlesource.com/emscripten-releases/+/refs/heads/main/src/build.py#673

scripts/update_python.py

@@ -108,7 +108,8 @@ def build_python():
         check_call(['brew', 'install', 'openssl', 'pkg-config'])
         if platform.machine() == 'x86_64':
             prefix = '/usr/local'
-            min_macos_version = '10.11'
+            # Keep in sync with CMAKE_OSX_DEPLOYMENT_TARGET in emsdk.py
+            min_macos_version = '10.14'
         elif platform.machine() == 'arm64':
             prefix = '/opt/homebrew'
             min_macos_version = '11.0'