BUILD: fix dragonfly build again on __read_mostly
It looks like some versions define it and others not. Better rely on
the macro itself rather than checking for a particular OS.
diff --git a/include/haproxy/compiler.h b/include/haproxy/compiler.h
index ca3b844..1f0c40d 100644
--- a/include/haproxy/compiler.h
+++ b/include/haproxy/compiler.h
@@ -89,7 +89,7 @@
#endif // USE_OBSOLETE_LINKER
/* use this attribute on a variable to move it to the read_mostly section */
-#if !defined(__DragonFly__)
+#if !defined(__read_mostly)
#define __read_mostly HA_SECTION("read_mostly")
#endif