[Vala] Vala broken on non-gnu systems

[pancake <pancake nopcode org>]

The commit a2s2120.. dated at 2011-02-10 adds the use of strnlen function in glib-2.0.vapi.

This function is a gnu extension on libc. This is not glib and this function is only available in GNU LibC. 
Do the build breaks on OSX, Windows and probably all BSD systems.

It would be great if someone with commit access (Jurg?) fixes that.

Thanks