Log Message:
Teach libX11 about cross-compilation:
- define an automake conditional for cross-compilation
- build xmakekeys in addition to makekeys when using
  cross-compilation. Provide manual compiliation rules to replace
  the default automake rules. We do use X11_CFLAGS, but we do *not*
  want CFLAGS as the latter can contain platform specific flags like
  -mieee on Alpha.
- Use either xmakekeys or makekeys depending on whether this is a native
  build or not.

Supported-by: Google SoC 2007