Convert EXA support preprocessor macro to USE_EXA

All the other DDXs do it this way.
Signed-off-by: Kevin Brace <kevinbrace@gmx.com>

configure.ac

@@ -121,7 +121,7 @@ AM_CONDITIONAL(XAA, test "x$XAA" = xyes)
 AC_MSG_RESULT([$XAA])
 
 # technically this should be a configure flag.  meh.
-AC_DEFINE(XF86EXA, 1, [Build support for Exa])
+AC_DEFINE(USE_EXA, 1, [Build support for EXA])
 
 SAVE_CPPFLAGS="$CPPFLAGS"
 CPPFLAGS="$CPPFLAGS $XORG_CFLAGS"

src/sis.h

@@ -230,7 +230,7 @@
 
 #ifdef SISVRAMQ
 #ifdef XORG_VERSION_CURRENT
-#if defined(SIS_HAVE_EXA) || (defined(XF86EXA) && (XF86EXA != 0))
+#if defined(SIS_HAVE_EXA) || (defined(USE_EXA) && (USE_EXA != 0))
 #if 1
 #define SIS_USE_EXA		/* Include code for EXA */
 #endif