Relax the requirements for a debug build

We required that a target be named 'debug-something' or to have at
least one of the configuration items debug_cflags and debug_lflags for
--debug to be accepted.

However, there are targets with no such markings but that will still
have debugging capabilities.  This is particularly true for mk1mf
builds, where the extra flags for debugging are figured out later on
by util/mk1mf.pl.

Reviewed-by: Rich Salz <rsalz@openssl.org>