Make --prenodef work more sensibly, remove caveat

--prenodef was causing the preprocessor-specific options (like -std=c99 for gcpp or -V199901L for mcpp) to be omitted. That makes no sense; '-p ext' should be used when custom options are desired (also modify the text that mentions it to make that clearer). Only the macros should be omitted.

The comment that warned about side effects with the order of --prenodef with respect to -p no longer applies, so remove it.

main.py

@@ -225,14 +225,11 @@ Options marked with * are used to define the preprocessor macros __AGENTID__,
 __AGENTKEY__, __AGENTIDRAW__, __AGENTNAME__, __ASSETID__ and __SHORTFILE__,
 and have no effect if --prenodef is specified.
 
-Using --prenodef before -p causes no macros whatsoever to be defined. If used
-after -p, the type casting macros string(...), key(...), etc. will be defined.
-
 If filename is a dash (-) then standard input is used.
 Use: {progname} -O help for help on the optimizer control options.
 
 Preprocessor modes:
-    ext       Invoke preprocessor
+    ext       Invoke a preprocessor with no default parameters
     mcpp      Invoke mcpp as preprocessor, setting default parameters pertinent
               to it. Implies --precmd=mcpp
     gcpp      Invoke GNU cpp as preprocessor, setting default parameters
@@ -583,7 +580,6 @@ def main(argv):
 
         # Build preprocessor command line
         preproc_cmdline = [preproc_command] + preproc_user_args
-        if predefines:
         if preproc == 'gcpp':
             preproc_cmdline += [
                 '-undef', '-x', 'c', '-std=c99', '-nostdinc',
@@ -596,6 +592,7 @@ def main(argv):
                 '-V199901L',
                 ]
 
+        if predefines:
             preproc_cmdline += [
                 '-Dinteger(...)=((integer)(__VA_ARGS__))',
                 '-Dfloat(...)=((float)(__VA_ARGS__))',