The Intel Compiler has these two command line flags to target specific instruction sets:
-xFOO (for example: -xSSE3, -xCORE-AVX2 or -xHost)
-axFOO (for example: -axSSE3,CORE-AVX2 - multiple comma separated arguments are valid)
When setting these compiler flags in CMAKE_CXX_FLAGS the ROOT build process chokes when making the PCH. This appears to be caused by forwarding these flags to "rootcling".
A patch which skips these command line flags for the PCH generation is attached.