make: cannot vfork() twice - on darwin

["Simon J. Gerraty" <sjg%crufty.net@localhost>]

This is "cute", only seems to impact darwin so far.
We get an error from vfork() if we've already done it
(this happens when trying to export a var that uses :sh in the child).

Index: main.c
===================================================================
RCS file: /cvsroot/src/usr.bin/make/main.c,v
retrieving revision 1.181
diff -u -p -r1.181 main.c
--- main.c      22 Apr 2010 20:25:16 -0000      1.181
+++ main.c      22 Apr 2010 22:24:34 -0000
@@ -1547,6 +1547,7 @@ Check_Cwd(const char **argv)
 char *
 Cmd_Exec(const char *cmd, const char **errnum)
 {
+    static int mypid = 0;
     const char *args[4];       /* Args for invoking the shell */
     int        fds[2];         /* Pipe streams */
     int        cpid;           /* Child PID */
@@ -1560,6 +1561,10 @@ Cmd_Exec(const char *cmd, const char **e
 
     *errnum = NULL;
 
+    pid = getpid();
+    if (!mypid)
+       mypid = pid;
+
     if (!shellName)
        Shell_Init();
     /*
@@ -1581,7 +1586,17 @@ Cmd_Exec(const char *cmd, const char **e
     /*
      * Fork
      */
-    switch (cpid = vfork()) {
+    if (pid == mypid) {
+       cpid = vfork();
+    } else {
+       /*
+        * We have already vfork'd
+        * we cannot do it again,
+        * at least on darwin.
+        */
+       cpid = fork();
+    }
+    switch (cpid) {
     case 0:
        /*
         * Close input side of pipe