configure: fix FreeBSD header detection (BZ#408)

This change is based on Xavier Beaudouin's patch (which fixes detection
of 3 config.h macros on FreeBSD without any impact to Linux build of
Quagga) and FreeBSD port patch (which fixes 5 config.h macros, but
breaks the Linux build), it fixes 5 macros and works for both FreeBSD 8
and Linux.
1 file changed