Add a --enable-debug-tcg option to configure

This patch allows DEBUG_TCGV to be defined (and also prevents NDEBUG
from being defined) when passing an option to the configure script.
This should help to prevent any accidental changes that enable
DEBUG_TCGV in tcg/tcg.h from being committed in future, and may
help to encourage testing with DEBUG_TCGV enabled.

Signed-off-by: Stuart Brady <stuart.brady@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7105 c046a42c-6fe2-441c-8c8c-71466251a162
