This macros checks if std::ref, added in C++11, is defined in the <functional> header.
If it is, define the ax_cv_cxx_have_ref environment variable to "yes" and define HAVE_CXX_REF.
NOTE: This macros depends on AX_CXX_NAMESPACES.
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Copyright © 2014 Enrico M. Crisostomo email@example.com
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