How better to implement strndup for MacOS versions that do not have it? (needed for EGL)

[barracuda156]

Two source file related to EGL use strndup which is missing on MacOS X < 10.7 – egl_has_extension_nocontext.c and egl_epoxy_api.c.

In Macports we can use legacysupport PortGroup, which provides needed definition: https://github.com/macports/macports-legacy-support/blob/master/include/string.h

I guess it can be borrowed and included either directly in those 2 files or in a header common to them. Condition can be something like:

#ifdef __APPLE__
#include <AvailabilityMacros.h>

#if __MAC_OS_X_VERSION_MIN_REQUIRED < 1070
[put definition here]
#endif
#endif