Suggested change in perl.h file

In perl.h there is a macro definition for the constant DEBUG. This
causes problems when other products need to include perl.c and also
define a macro DEBUG.

For example, PostgreSQL plperl has this problem.

Perl would probably "play" better with other products if it defined
the macro as PERLDEBUG rather than just DEBUG.

• Suggested change in perl.h file by David R. Favor