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.
Add ability to "feed" try_compile (and try_run) sources more directly,
either from literal content, or from a CMake variable which contains
literal content. This saves the user from needing a separate step to
write the content to a file, and allows for the sources to only exist in
the scratch directory.
