Add -O3 to all C/C++ compiles.

So far, we are compiling C/C++ code without any optimization.

In non-representative, but practically relevant tests, the -O3
improved the total query time for a demanding graph by ~20%.

Change-Id: I5e8123650e53a4933ed4fbe63d0b1ca67217b865
Merge "Add -O3 to all C/C++ compiles."