createProcess ENOENT regression on mac OS

This may be the same issue as #224, but it's on mac OS so I thought I'd mention it separately. I tried to test out the fix at https://github.com/haskell/process/pull/232, but I got a segfault and couldn't figure out if it's because I compiled it wrong or if it's actually broken. Anyway, for completeness:

ghc 8.8.3, process-1.6.8.0:

```
Prelude> import System.Process
Prelude System.Process> createProcess (proc "aoeu" [])
*** Exception: aoeu: createProcess: runInteractiveProcess: exec: does not exist
(No such file or directory)
```

ghc 9.2.1, process-1.6.13.2:

```
Prelude> import System.Process
Prelude System.Process> createProcess (proc "aoeu" [])
*** Exception: aoeu: createProcess: posix_spawnp: failed (Undefined error: 0)
```

ghc 8.8.3, process 1.6.14.0, via cabal v1-repl:

```
*System.Cmd> import System.Process
*System.Cmd System.Process> createProcess (proc "aoeu" [])
[1] 98747 segmentation fault cabal v1-repl
```

I also noticed that `./configure` mentioned this:
```
checking whether posix_spawn reports errors sensibly... yes
```

This makes me think either darwin is non-sensible in some other way, or this is not #224 after all.

Start by comparing this report with issue #224 and pull request #232, then reproduce the createProcess failure on macOS using the reported GHC and process versions. Inspect the ./configure check for whether posix_spawn reports errors sensibly and the createProcess path involved. Done means the macOS behavior is understood and no longer produces the reported incorrect error or segmentation fault.

Written by the indexing model from the issue text.