This is the "maximum" assignment operator, a GCC extension.
If the extension is not enabled, then you will not be able to use this feature.
As of 4.0.1:
The G++ minimum and maximum operators ( and >?) and their
compound forms () and >?=) have been deprecated and will be
removed in a future version. Code using these operators should be
modified to use std::min and std::max instead.
It looks like they were gone by 4.0.4.
