configure: Don't enable valgrind unless specifically asked for

Running "make check" is quite slow if valgrind is enabled, and most of the time failures is the result of a leak in the GL driver. Since when building packages testing is enabled, this will make the package build fail when valgrind is installed. So instead of automagically enabling valgrind when it is installed, make it a configure option that also may fail if valgrind is not installed, but requested.