config: Check for and use gcc -Wlogical-op and -Wduplicated-cond.

config: Check for and use gcc -Wlogical-op and -Wduplicated-cond.

Both -Wlogical-op and -Wduplicated-cond may produce useful warnings.
But we have to check them first. Older versions of gcc had a -Wlogical-op
that warned on some constructs using macros that are not erronious.
Only GCC6 has -Wduplicated-cond.

Signed-off-by: Mark Wielaard <mjw@redhat.com>