build: update GCC warnings

build: update GCC warnings

* configure.ac: Remove the -Wsuggest-attribute=pure
enablement on GCC >= 4.7, as that was moot since
gnulib was already enabling that warning in its default set.
The false positive was seen with 4.6.2, but confirmed
not present in 4.6.3, so that's sufficiently old to
just leave this enabled unconditionally.

Remove the -Wsuggest-attribute={const,noreturn}
enablement, as gnulib already has those in the default set.

Enable the -Wlogical-op warning for GCC >= 4.8.0
as that is confirmed OK with coreutils at least, due to
fixing: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43772

Remove the -Wno-logical-op override since the main
-Wlogical-op flag is now sufficient to control this warning
as of GCC 4.6.3 at least.