Warn rather than crash on preprocessor failure.

Detect likely failure of the DynASM preprocessor
during configuration and emit a warning line into
the Makefile rather than a command that would crash.

Configure.pl

@@ -162,14 +162,19 @@
 }
 
 if ($args{'enable-jit'}) {
+    if (system($config{lua} .' -e \'require("bit");\'') == 0) {
+        $config{dynasm_rule} = '$(CMD)$(LUA) ./3rdparty/dynasm/dynasm.lua -o $@ $<';
+    }
     if ($Config{archname} =~ m/^x86_64/) {
         $config{arch} = 'x86_64';
         $config{jit} = '$(JIT_X64)';
     } else {
         say "JIT isn't supported on $Config{archname} yet.";
     }
 }
+
 # fallback
+$config{dynasm_rule} //= '$(MSG) "Warning: Cannot run preprocessor (try reconfigure)"';
 $config{arch} //= 'stub';
 $config{jit} //= '$(JIT_STUB)';
 

build/Makefile.in

@@ -491,7 +491,7 @@ switch no-tracing no-cgoto:
 
 %.c: %.dasc
 	$(MSG) preprocessing $< to $@
-	$(CMD)$(LUA) ./3rdparty/dynasm/dynasm.lua -o $@ $<
+	@dynasm_rule@
 
 src/main@obj@: src/main.c
 	$(MSG) compiling $@