travis: measure coverage of C with -O0
travis: measure coverage of C with -O0

Some functions are optimized out in -O2 so they
are marked as uncovered by tests if -O2 is enabled.

starius/lua-npge,npge/lua-npge,starius/lua-npge,npge/lua-npge,npge/lua-npge,starius/lua-npge