build-sys: add a rule to print a variable

$ make print-CFLAGS
CFLAGS=-fsanitize=address -Og -g

Trick from various sources:
https://stackoverflow.com/questions/16467718/how-to-print-out-a-variable-in-makefile
https://www.cmcrossroads.com/article/printing-value-makefile-variable



Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180104160523.22995-4-marcandre.lureau@redhat.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>