See SERVER-5702 and https://github.com/mongodb/mongo/commit/82544c7d08c033184104a5c9ab115e4047fdbdbd

In short, the build system knows how to build a file, build/unittests.txt, and does build it as part of the "all" rule. The file contains a list of unit test programs that should be run, with no arguments, and whose exit code is 0 to indicate test passage, and non-zero to indicate failure.

I'd recommend implementing a "unittests" rule to smoke – "smoke unittests" would run the programs indicated.