configure.in sets LDSHARED to "ld -b" around line 1477. Whether this is a bug, I don't know - there may have been HP-UX systems where this was the proper way of doing things.

These days, on most systems (not sure whether this includes HP-UX), direct linking with ld is discouraged; one should use the C compiler for linking.

Unless somebody steps in and can tell the full story, I would advise to use $(CC) for linking on HPUX if the compiler is gcc (see SunOS processing as to how to determine gcc).