#286 Add support for cross-compilation under Linux

[Gregory Lemercier]

Add support for cross-compilation under Linux, used for compiling Irrlicht for embedded devices such as the ARM based i.MX6 Sabre SD board.

The patch basically modifies the Makefiles this way:
- Add "NX11" variable to disable linking to the X libraries (not available on most embedded devices)
- Don't override OGLESLIBS variable if already defined, so that users can specify their own OGLESLIBS if this one is specific (also the case for most embedded platforms that may have different GPUs, therefore different libraries to link to)

This patch should not affect current behavior of the build system as long as those variables are not set. It modifies the Makefile of libIrrlicht and also the 10 first example projects. If the approach is validated, we can then report the changes to the other example projects.