Fixups for the android/bionic build probes patch.

Fixups for the android/bionic build probes patch.

The CFG_* symbols are always defined so if() can use them as compile-time
constants, so don't if defined() them.

Doing USE_BLAH() around variable definitions opens up the same potential for
config-dependent build breaks as #ifdefs do, just make the whole command
depend on the symbol for now, factor out the utmpx infrastructure later.

The PTY probe was always failing because it used NULL without #including
the header that defines it. Substitute 0 instead.