Small inconsistencies in configure checks

Hi,

JFYI. I am not sure it's worth reporting but, when building 2.5.5 for ALT
I noticed small inconsistencies in configure output.

1. xapian-binding:

  checking for /usr/bin/rdoc... no
  checking for rdoc... /usr/bin/rdoc

Looks curious but no problem since it's found anyway.

2. xapian-core when built with GCC:

  checking whether __builtin_add_overflow is declared... yes
  ...
  checking whether _addcarry_u64 is declared... no

There is actually _addcarry_u64 in GCC too, but it's in x86intrin.h
instead of intrin.h. This is no problem either, since it's superimposed
by __builtin_add_overflow anyway.

Thanks,