enforce max C89 compatibility on gcc (RT#87069)

Add '-DPERL_GCC_PEDANTIC -std=c89 -pedantic-errors -Wno-long-long' to
CCFLAGS when the compiler is gcc to enforce the maximum C89
compatibility possible for other compilers.
release 0.49
remove #warning (for -pedantic-errors) (RT#87398)

The #warning pragma is not standard C89 and throws an error under
-pedantic-errors.
release 0.50