[oe] Specify X11 include and libs directories

[Joaquim Duran]

Dear all,

I'm writting a .bb file for FLTK version 1.1.7. While executing the 
./configure script, I should specify the include and lib directories of 
X in the cross development.

I know how specify the parameter to ./configure script, but the problem 
is that I don't know how specify these directories using variables to be 
flexible and from which folder from BUILD dir.

How can I specify them? Any help will be appreciated.

Thanks and Best Regards,
Joaquim Duran