pgsql: Fix unportable usage of __has_attribute

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1fa22a43a56e1fe44c7bb3a3d5ef31be5bcac41d

Modified Files
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src/include/c.h | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)