The native (bundled) compiler for HPUX currently will not work,
because it is not a full ANSI C compiler.  The optional ANSI C
compiler should work as long as you give it the -Ae flag
(i.e. ./configure CC='cc -Ae').  This is equivalent to
./configure CC='c89 -D_HPUX_SOURCE', which was the previous
recommendation.  This has only been tested recently for HPUX 10.20.