strtoimax, strtoumax: simplify, port to HP-UX 11.00 64-bit

strtoimax, strtoumax: simplify, port to HP-UX 11.00 64-bit

This supports apps that need pointers to strtoimax and strtoumax,
and ports to HP-UX 11.00 64.bit, which has macros that expand to
nonexistent functions.  See
<http://lists.gnu.org/archive/html/bug-gnulib/2011-04/msg00241.html>
et seq.
* lib/inttypes.in.h (strtoimax, strtoumax): #undef before declaring.
* m4/strtoimax.m4 (gl_FUNC_STRTOIMAX): Don't check whether it's
a macro.
* m4/strtoumax.m4 (gl_FUNC_STRTOUMAX): Likewise.