Explicitly specify where tcmalloc comes from (#235)
Explicitly specify where tcmalloc comes from (#235)

This change adds a declaration that `libtcmalloc_minimal.a` is generated
by the external project. This is needed by the Ninja generator to
properly build.

To reproduce:
run `cmake -GNinja path/to/quickstep`
then `cmake --build path/to/build/dir` and see it fails.
pivotalsoftware/quickstep,pivotalsoftware/quickstep,pivotalsoftware/quickstep,pivotalsoftware/quickstep