Fix and clean code which tests the gcc version

The code which tests whether gcc supports warn_unused_result was wrong.
Remove the wrong test from configure and replace it by code using
macro QEMU_GNUC_PREREQ in compiler.h.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

compiler.h

@@ -18,7 +18,7 @@
 
 #define QEMU_NORETURN __attribute__ ((__noreturn__))
 
-#ifdef CONFIG_GCC_ATTRIBUTE_WARN_UNUSED_RESULT
+#if QEMU_GNUC_PREREQ(3, 4)
 #define QEMU_WARN_UNUSED_RESULT __attribute__((warn_unused_result))
 #else
 #define QEMU_WARN_UNUSED_RESULT

configure

@@ -2359,23 +2359,6 @@ if compile_prog "" "" ; then
     need_offsetof=no
 fi
 
-##########################################
-# check if the compiler understands attribute warn_unused_result
-#
-# This could be smarter, but gcc -Werror does not error out even when warning
-# about attribute warn_unused_result
-
-gcc_attribute_warn_unused_result=no
-cat > $TMPC << EOF
-#if defined(__GNUC__) && (__GNUC__ < 4) && defined(__GNUC_MINOR__) && (__GNUC__ < 4)
-#error gcc 3.3 or older
-#endif
-int main(void) { return 0;}
-EOF
-if compile_prog "" ""; then
-    gcc_attribute_warn_unused_result=yes
-fi
-
 # spice probe
 if test "$spice" != "no" ; then
   cat > $TMPC << EOF
@@ -2998,9 +2981,6 @@ fi
 if test "$need_offsetof" = "yes" ; then
   echo "CONFIG_NEED_OFFSETOF=y" >> $config_host_mak
 fi
-if test "$gcc_attribute_warn_unused_result" = "yes" ; then
-  echo "CONFIG_GCC_ATTRIBUTE_WARN_UNUSED_RESULT=y" >> $config_host_mak
-fi
 if test "$fdatasync" = "yes" ; then
   echo "CONFIG_FDATASYNC=y" >> $config_host_mak
 fi