linux: add initial support for ARM cross-compilation
Add config file for linux arm cross-compilation
x264: fix linux arm build
libvpx: don't use hardoced gcc
libmad: fix linux arm build
variants: support 'no' modifier for variants

Use positive variants and allow a 'no' modifier for them
variants: add new variants to customize builds even more

diabled for ios and android:
  gtk, python, clutter
enabled for linux:
  x11, alsa, pulse, cdparanoia, v4l2
theora: disable docs and tests in cross-compilation
linux: set armv7 cflags correctly