MFH: r431816

lang/spidermonkey24: prefer pre-C++11 decltype from libc++

libc++ 4.0 started to use it within <string> in a way incompatible
with typeof which broke build but upstream switched to C++11 long
ago to notice -Wmacro-redefined.

PR: 216010
Approved by: ports-secteam (junovitch)