Makefile: Add check of version conflict of a2ping

This conflict is likely to be encountered by new readers.
Checking it in Makefile and output reference to FAQ-BUILD.txt
should be of help.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
1 file changed