Default to no compiler optimization for extensions

clang++ miscompiles the stanc parser at high optimization levels (see https://groups.google.com/d/msg/stan-users/HP3YjPrBKO0/VVuJyE3Gs-EJ).

Fortunately, stanc isn't the thing that needs to be fast.

Since libstan is not affected by this issue, this commit splits out
the compiler flags for libstan from those for the extensions (mostly
stanc), and backs the latter down to -O0.