Introduce a new signature for try_compile (and try_run) which removes
the `bindir` argument and requires the SOURCES tag. This will eventually
allow us to add other ways of providing sources, but also allows us to
change the behavior without breaking compatibility.
The old signature uses a special, but non-unique temporary location
inside the specified `bindir`, which conventionally is just the
project's build directory. The new signature unconditionally uses the a
unique temporary directory which is unconditionally within the project's
build directory (which is no longer separately specified). This ensures
that successive runs do not overwrite previous runs, will simplify
debugging, and should also, eventually, allow us to execute multiple
trials in parallel.
