With this it is now possible to do
foobar = executable('foobar', ...)
meson.override_find_program('foobar', foobar)
Which is convenient for a project like protobuf which produces both a
dependency and a tool. If protobuf is updated to use
override_find_program, it can be used as
protobuf_dep = dependency('protobuf', version : '>=3.3.1',
fallback : ['protobuf', 'protobuf_dep'])
protoc_prog = find_program('protoc')