isinf is a macro on HP-UX. Currently, configure.in uses AC_CHECK_FUNCS, 
which checks for functions only, to check for isinf. I've attached a 
patch that replaces the current test with a test that includes math.h.

The HP-UX manual page says "The ISO/ANSI C committee has approved the 
isinf() macro for inclusion in the C9X draft standard. [...] The 
isinf() macro replaces the isinf() and isinff() functions, which are 
obsolete and are no longer supported."

It seems that C99 defines the following macros: fpclassify(), 
isfinite(), isinf(), isnan(), isnormal(), signbit().