Unix configure: fix compilation with recent MySQL versions (5.6 for example), related to a symbol no longer declared in my_sys.h (#5284)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26582 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx