Allow compile-time culling of verbprintf() calls

This introduces a new macro, MAX_VERBOSE_LEVEL, which culls
all verbprintf() calls above that verb_level at compile time, so
that they never make it into the compiled code at all.

On x86 you can barely notice the performance difference from this,
but on ARM (especially with old GCC), varargs stuff really sucks
a lot, and just calling verbprintf() at all (even to return
immediately) is actually very expensive.
Merge pull request #7 from arekinath/no_verbprintf

Allow compile-time culling of verbprintf() calls