libxl, configure: print a warning if flex/bison are needed

libxl, configure: print a warning if flex/bison are needed

This patch adds better support for both Flex and Bison, which might
be needed to compile libxl. Now configure script sets BISON and FLEX
Makefile vars if bison and flex are found, but doesn't complain if
they are not found.

Also, added some Makefile soccery to print a warning message if
Bison or Flex are needed but not found.

[ Improved the warning message slightly. -iwj ]

Signed-off-by: Roger Pau Monne <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>