Fix unportable usage of __has_attribute
This should fix the breakages caused by
697e1d02f, which seems to break
the build for GCC version < 5.
It seems, according to the GCC manual that __has_attribute is a "special
operator" and must be tested without any other conditions in the
preprocessor test.
Per recommendation from the GCC manual via Greg Nancarrow
Reported-by: Greg Nancarrow
Discussion: https://postgr.es/m/CAJcOf-euSu8fhC10v476o9dqnjqKysVs1_vRms-_fvajpZ3kFw@mail.gmail.com