Disable use of vfork()

vfork() is just too weird.  For instance, in this build:

  http://hydra.nixos.org/build/3330487

the value fromHook.writeSide becomes corrupted in the parent, even
though the child only reads from it.  At -O0 the problem goes away.
Probably the child is overriding some spilled temporary variable.

If I get bored I may implement using posix_spawn() instead.

configure.ac

@@ -116,7 +116,7 @@ AC_CHECK_HEADERS([sys/mount.h], [], [],
 
 
 # Check for vfork.
-AC_FUNC_FORK()
+#AC_FUNC_FORK()
 
 
 # Check for lutimes, optionally used for changing the mtime of

src/libstore/build.cc

@@ -355,7 +355,7 @@ void Goal::trace(const format & f)
 
 
 /* Common initialisation performed in child processes. */
-void commonChildInit(Pipe & logPipe)
+static void commonChildInit(Pipe & logPipe)
 {
     /* Put the child in a separate session (and thus a separate
        process group) so that it has no controlling terminal (meaning