Provide processor feature-set (instruction-set-extensions) support via USE-flags

[kripton@…]

I try to build a Gentoo-based system using MyhtTV in a virtual-machine (VirtualBox?) on an AMD Phenom II X6-based host. Later, I'll copy the system to the actual target which is a Celeron-M based on the PentiumIII (Coppermine). The target CPU only supports MMX and SSE. The host CPU supports much more (SSE2, SSE3, ...) and MythTV's build system (I use the ebuilds from ​https://github.com/MythTV/packaging/tree/master/Gentoo) detects the CPU's features at compile-time. On the target, mythfrontend gets killed with SIGILL (Illegal instruction). Either MythTV should use run-time CPU detection (quite some work to implement) or the ebuilds should have USE-flags to hard-force the instruction set extensions that the target-processor supports (Not so much work, there are many ebuilds in the official tree that do so and since you have the configure-options anyways...)