Though we were compiling cleanly with clang, using gcc8 picked
up several nits, which might bite this port, when building on
the platforms, where we still use gcc.

Also, when using port-installed gcc8, ${LOCALBASE}/include is
implicitly used to search for headers BEFORE /usr/include. This
may cause gdbm-provided ndbm.h to be picked up instead of the
base version, and we're not ready for that.

The already-built packages should be unaffected, hence, still
no PORTREVISION bump...

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