build in non-debug mode by default

* declare NDEBUG for library and contrib
* activate pjsip extra checks as all PJ_ASSERT's are deactivated

Side effect: pjsip does not call abort() due to disabled assertions.

If debug is required:
* contrib: call bootstrap with --enable-debug
* libring: call configure with --enable-debug

Change-Id: I47b104c0f32352f0503457a9d419c4ed5db4e126