try_compile: Rename SOURCE_FROM_ARG -> SOURCE_FROM_CONTENT
Change the SOURCE_FROM_ARG
keyword to try_compile to SOURCE_FROM_CONTENT
(which we can do because it was recently added and hasn't been in a release yet). The new name should be clearer as to what it does, and also more consistent with the CONTENT arguments to some other commands.
Also, fix a typo in an error message.