I have been investigating this and have come to the conclusion that:

a) this needs to be defined for mesalib (for glx only, obviously) on
64-bit platforms.

b) This _should_ be defined in modular-xorg-server (by the server,
e.g. added--by the configure script--to GLX_CPPFLAGS (if that exists,
else GLX_CFLAGS)  on 64-bit platforms, but it got lost in the modular
transition due to inadequate testing.

My conclusion is further reinforced by the fact that our build system
defines it. ;)

--Blair
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