sqlite: Enable optimization

Commit a28940d9 changed the SQLite
build to use CFLAGS instead of NIX_CFLAGS_COMPILE, but that's really
bad because it clobbers the default -O2 flag. So all this time we had
an unoptimized SQLite build. (This is one of the reasons why
NIX_CFLAGS_COMPILE exists - messing with CFLAGS is almost never a good
idea.)