CMake not detecting ${GNUCC_ARCH} on Arch Linux, gcc 9.2

I have just cloned Hyperscan to my machine and am building it. Os is Arch Linux with latest kernel and GCC and I had some trouble with CMake. It repported error with Unix Makefiles generator. It didn't found the right architecture for -mtune flag.

The macro was in CMakeLists.txt, line 196:
` set (EXEC_ARGS ${CC_ARG1} -E - -mtune=${GNUCC_ARCH})`

Changing this to:
`set EXEC_ARGS ${CC_ARG1} -E - -mtune=native)`

seems to fix the issue. At least it did for me, Hyperscan seems to have built fine.