fix warning treated as error, caused by using CFLAGS -Wall -Werror

I came across this error while building varnish 4.0.0, having subtree-merged libvmod-querystring into the varnish tree; the latter uses CFLAGS -Wall -Werror by default.

Maybe there is a better way to handle this warning, but I just wanted to let you know about it.

While at it, I changed CFLAGS to AM_CFLAGS so as to not override the above.