config.guess fails on HP-UX

[Dietmar Schindler]

GCC version 2.95.1
system type hppa2.0 hpux10.20

config.guess returns "-hp-hpux10.20" instead of "hppa2.0-hp-hpux10.20".
This is because the C program dummy generated temporarily by
config.guess doesn't work when a certain macro isn't defined for the
compilation. The bug can be fixed by changing line 409 of config.guess:

        (${CC-cc} dummy.c -o dummy 2>/dev/null ) && HP_ARCH=`./dummy`

to

        (${CC-cc} -D_HPUX_SOURCE dummy.c -o dummy 2>/dev/null ) &&
HP_ARCH=`./dummy`