custom_targets can have conflicting depfiles

[dcbaker]

Describe the bug
Two custom_targets in the same directory with the same depfile will successfully configure, but will write to the same location, possibly racing, and confusing ninja

To Reproduce

foreach t : ['a', 'b']
  custom_target(..., depfile : 'dep.d')
endforeach

Expected behavior
Either Meson would have an error (like it would if two targets have the same outputs) or that Meson would write the depfiles into the target's private directory, avoiding such issues, or both.

system parameters

• what meson --version 1.6.0