package/efl/libevas: x-includes and x-libraries must be set for cross-compiling

package/efl/libevas: x-includes and x-libraries must be set for cross-compiling

evas_engine_software_xlib_cflags and evas_engine_software_xlib_libs
contains unsafe libraries paths if x-include and x-libraries are
not set in libevas.mk.

config.log:
evas_engine_software_xlib_cflags='-I/usr/X11R6/include'
evas_engine_software_xlib_libs='-L/usr/X11R6/lib -lX11 -lXext'

Fixes:
http://autobuild.buildroot.net/results/e5f/e5fb1e62cb634b20233751b4ea3b0630de70e9e0/

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>