libecore: x-includes and x-libraries must be set for cross-compiling

If those flags are not explicitly passed, the libecore configure
script will include -I/usr/X11R6/include and -L/usr/X11R6/lib in the
compile flags, which are obviously unsafe for cross-compilation.

The fix is similar to "package/efl/libevas: x-includes and x-libraries
must be set for cross-compiling" done by Romain Naour on libecore.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>