set CONFIG_OS2 using new method

Originally committed as revision 7092 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -443,6 +443,7 @@ HAVE_LIST='
     malloc_h
     memalign
     mlib
+    os2
     os2threads
     pthreads
     sdl
@@ -1956,10 +1957,6 @@ enabled_any pthreads beosthreads os2threads w32threads && enable threads
 print_config HAVE_   $TMPH config.mak $HAVE_LIST
 print_config CONFIG_ $TMPH config.mak $CONFIG_LIST
 
-if test "$os2" = "yes" ; then
-  echo "#define CONFIG_OS2 1" >> $TMPH
-fi
-
 if test "$targetos" = "Darwin"; then
   echo "#define CONFIG_DARWIN 1"  >> $TMPH
 fi