When using the GNU header files on a system, the icc compiler emulates
the behavior of the gcc compiler on the system. (See https://
software.intel.com/en-us/node/522750) This appears to mean that icc
features that are not available in gcc could get turned off. In
particular, we found that the __assume feature stopped working.

To get around this problem, do not use __assume when compiling with icc
and __GNUC__ is defined. Instead, use the available gcc features.