Refer to xelatex and latex options via variables. This allows the user
to override the pdflatex and latex options to use on the make command
line for experimenting. As a side effect, this makes the makefile a bit
tidier.
Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>