Add compiler depenend magic to keep the version compiled with option "debug"
usable with modern gcc.
Since the full "debug" version will behave differently to the standard
version (as it enables all the mozilla internal consistency checks, and
also drops compiler optimization), it is not very usefull when trying to
debug crashes that could be compiler bugs, or mozilla low level bugs -
so provide a new option "debug-info" that creates a debuggable, but
fully optimized version.
The result is best run from the pkgobj dir via the
work/build/dist/bin/run-mozilla script with options "-g ./firefox".
No changes to the default pkg generated.

(martin)