using @OUTPUT@ in a custom_target depfile argument results in double path

[dcbaker]

Describe the bug
In a subdirectory, create a custom target that writes a depfile (say in 'include)the depfile passed to the executable will have the path twice, soinclude/include/depfile.dinstead ofinclude/depfile.d`

To Reproduce
In a subdirctory. This will not trigger from the root meson.build flie

custom_target(
  command : ['a command', '--depfile', '@DEPFILE@', '-o', '@OUTPUT@'],
  output : 'foo',
  depfile : '@[email protected]',
)