Add mechanism to avoid deleting temporary temporary directories

[jbauman42]

BUILD_TMPDIR and EXT_BUILD_DEPS are deleted before and after running the build command, which means that rebuilds of the external project need to build every single file, which can be expensive for larger projects.

It would be nice if there was an option to avoid deleting those directories; that would increase the risk of correctness problems, but could be useful for development purposes.