Log Message:
Don't pass "--with-macos-sdk=/nonexistent". Fixes this error on Catalina
with Xcode 12:

configure: error: SDK not found.  When using --with-macos-sdk, you must
specify a valid SDK.  SDKs are installed when the optional cross-development
tools are selected during the Xcode/Developer Tools installation.

While here, merge some of the Darwin conditional logic.