libbb: fix no MMU build

uClibc-ng defines __ARCH_HAS_MMU__ for ARM or m68k because these
architectures may potentially have MMU. The code should test for
__ARCH_USE_MMU__, which indicates if the MMU is really in use. But
since the configure script detects the presence of fork(), using
HAVE_FORK directly is a better solution.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
---
Patch status: no active upstream

diff -Nuar ipkg-0.99.163-orig/libbb/libbb.h ipkg-0.99.163/libbb/libbb.h
--- ipkg-0.99.163-orig/libbb/libbb.h	2006-02-06 10:13:02.000000000 +0200
+++ ipkg-0.99.163/libbb/libbb.h	2016-07-26 13:50:09.296988855 +0300
@@ -341,7 +341,7 @@
 #define CONSOLE_DEV "/dev/console"
 
 /* Cope with mmu-less systems somewhat gracefully */
-#if defined(__UCLIBC__) && !defined(__ARCH_HAS_MMU__)
+#if !defined(HAVE_FORK)
 #define fork	vfork
 #endif