`cabal build --enable-profiling` should use a separate build directory

It is quite common to alternate normal and profiling builds while developing. Currently, cabal uses the same build directory for both, which results in a lot of rebuilding. The current workaround is to manually specify separate build directories for each type of build, which works but is noisy.

This seems easy to fix: use some other build directory by default if `--enable-profiling` is present (e.g., `dist-profiling`). Users can still use `--builddir` to pick a different one, if they'd like.