Make it possible to to do test builds for NOMMU architectures

And while using it, I discovered that -D doesn't work too well
on NOMMU. Added a comment about it.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/defs.h b/defs.h
index c9200fa..e33c420 100644
--- a/defs.h
+++ b/defs.h
@@ -123,6 +123,9 @@
  * This needs Linux kernel 3.4.x or later to work.
  */
 #define USE_SEIZE 1
+/* For forcing NOMMU build, set to 1 */
+#define NOMMU_SYSTEM 0
+
 
 #if defined(MIPS) && _MIPS_SIM == _MIPS_SIM_ABI32
 # define LINUX_MIPSO32
@@ -211,7 +214,7 @@
 # define PTRACE_POKEUSER PTRACE_POKEUSR
 #endif
 
-#ifdef USE_SEIZE
+#if USE_SEIZE
 # undef PTRACE_SEIZE
 # define PTRACE_SEIZE		0x4206
 # undef PTRACE_INTERRUPT