Suppress ZMQ_EXTENDED_CONSTEXPR on gcc with partial C++14 support

[gin-ahirsch]

cf. https://gcc.gnu.org/projects/cxx-status.html#cxx14

[gin-ahirsch]

Had to change from checking __cpp_constexpr to a specific gcc version. Looks like gcc <= 5.3 errors, but still defines __cpp_constexpr to 201304 so that's insufficient.

[coveralls]

Pull Request Test Coverage Report for Build 2280258814

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 86.037%

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Totals
Change from base Build 1383071410:	0.0%
Covered Lines:	801
Relevant Lines:	931

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💛 - Coveralls

[gummif]

Thanks