atomic_add_unless() in libdrm xf86atomic.h may reverse the meaning
of its return value.

Linux kernel documentation seems to indicate something like:
"Returns non-zero if @v was not @u, and zero otherwise."

A simple inverting the meaning of libdrm's return value
allowed glxgears to properly function for a hacked version
of pkgsrc on DragonFly 4.7-DEVELOPMENT on an Intel IvyBridge
integrated graphics machine.  glxgears was already properly
functioning on the same machine for NetBSD current, NetBSD
using its own atomic operations declared in its sys/atomic.h
header.

A one line (character) fix is of the form:

--- xf86atomic.h.orig   2015-09-22 04:34:51.000000000 +0000
+++ xf86atomic.h
@@ -111,7 +111,7 @@ static inline int atomic_add_unless(atom
        c = atomic_read(v);
        while (c != unless && (old = atomic_cmpxchg(v, c, c + add)) != c)
                c = old;
-       return c == unless;
+       return c != unless;
 }