Fix a problem that causes build failures with CMake on Yosemite.

Change-Id: I52e81656470df887eb3c53b2632bf5a310b1b063
Reviewed-on: https://code.wireshark.org/review/5229
Reviewed-by: Guy Harris <guy@alum.mit.edu>
(cherry picked from commit b2c2debd978518c5c39781567a0e00f6713cc94e)
Reviewed-on: https://code.wireshark.org/review/5231