meson: only use lists of commands in cross file if required
There's a bug in Meson[1] where it find_program("foo") will fail if foo is
defined in the cross file as a list.
This is causing the Meson build of libdrm to fail, but for this instance we can
work around the problem by only using lists in the cross file if there are
arguments, and just using a string if there are not.
