floss: Fix build ignoring new C++ libraries

Cargo was not rerunning build.rs for linking when there were new C++
libraries available. This adds a rerun-if that checks the C++ output
directory for changes when making this decision.

Bug: 203696027
Tag: #floss
Test: ./build.py with C++ changes only and confirm changes applied
Change-Id: I46c968ce92ffddb7538b31df2103b79b0377fe3a