Ensure binaries are compatible with OS X 10.9

Author: jlaine

.github/workflows/build.yml

@@ -29,6 +29,7 @@ jobs:
           CIBW_BEFORE_BUILD: scripts/build-openssl /tmp/vendor
           CIBW_BEFORE_BUILD_WINDOWS: scripts\build-openssl.bat C:\cibw\vendor
           CIBW_BUILD: cp38-*
+          CIBW_ENVIRONMENT_MACOS: CFLAGS=-mmacosx-version-min=10.9
           CIBW_TEST_COMMAND: python -c "import dummy"
         run: |
           pip install cibuildwheel