Enable -Wall by default.

Fixes Bugzilla #1284.

(But probably upsets buildbot.  :)  )
(transplanted from 516c5c4e7beabd27608c67cab5b0259643a9083c)

--HG--
branch : SDL-1.2
extra : transplant_source : Ql%5CN%7B%EA%BD%27%60%8Cg%CA%B5%B0%25%96C%A9%08%3C

configure.in

@@ -875,6 +875,28 @@ CheckVisibilityHidden()
     fi
 }
 
+dnl See if GCC's -Wall is supported.
+CheckWarnAll()
+{
+    AC_MSG_CHECKING(for GCC -Wall option)
+    have_gcc_Wall=no
+
+    save_CFLAGS="$CFLAGS"
+    CFLAGS="$save_CFLAGS -Wall"
+    AC_TRY_COMPILE([
+    int x = 0;
+    ],[
+    ],[
+    have_gcc_Wall=yes
+    ])
+    AC_MSG_RESULT($have_gcc_Wall)
+    CFLAGS="$save_CFLAGS"
+
+    if test x$have_gcc_Wall = xyes; then
+        EXTRA_CFLAGS="$EXTRA_CFLAGS -Wall"
+    fi
+}
+
 
 dnl Do the iPod thing
 CheckIPod()
@@ -2785,6 +2807,9 @@ case "$host" in
         ;;
 esac
 
+dnl Do this on all platforms, after everything else.
+CheckWarnAll
+
 # Verify that we have all the platform specific files we need
 
 if test x$enable_joystick = xyes; then