osproc: Define and select spawn/fork same way

Previously startProcessAuxFork was defined even when it was unused, also the
compile time condition to select spawn vs fork was not always the same for
definitions and calls.

This change just defines the condition in one place as constant, then in other
places we just check for `when useProcessAuxSpawn`.

lib/pure/osproc.nim

@@ -614,9 +614,12 @@ elif not defined(useNimRtl):
     optionPoStdErrToStdOut: bool
   {.deprecated: [TStartProcessData: StartProcessData].}
 
-  when not defined(useFork):
+  const useProcessAuxSpawn = declared(posix_spawn) and not defined(useFork) and
+                             not defined(useClone) and not defined(linux)
+  when useProcessAuxSpawn:
     proc startProcessAuxSpawn(data: StartProcessData): Pid {.
       tags: [ExecIOEffect, ReadEnvEffect], gcsafe.}
+  else:
     proc startProcessAuxFork(data: StartProcessData): Pid {.
       tags: [ExecIOEffect, ReadEnvEffect], gcsafe.}
   {.push stacktrace: off, profiler: off.}
@@ -674,9 +677,7 @@ elif not defined(useNimRtl):
     data.optionPoStdErrToStdOut = poStdErrToStdOut in options
     data.workingDir = workingDir
 
-
-    when declared(posix_spawn) and not defined(useFork) and
-        not defined(useClone) and not defined(linux):
+    when useProcessAuxSpawn:
       pid = startProcessAuxSpawn(data)
     else:
       pid = startProcessAuxFork(data)
@@ -706,7 +707,7 @@ elif not defined(useNimRtl):
       discard close(pStdin[readIdx])
       discard close(pStdout[writeIdx])
 
-  when not defined(useFork):
+  when useProcessAuxSpawn:
     proc startProcessAuxSpawn(data: StartProcessData): Pid =
       var attr: Tposix_spawnattr
       var fops: Tposix_spawn_file_actions
@@ -752,7 +753,7 @@ elif not defined(useNimRtl):
       discard posix_spawnattr_destroy(attr)
       chck res
       return pid
-
+  else:
     proc startProcessAuxFork(data: StartProcessData): Pid =
       if pipe(data.pErrorPipe) != 0:
         raiseOSError(osLastError())