configure: fix -enable-debug with newer toolchains

Fedora 17's toolchain wants optimization enabled for _FORTIFY_SOURCE;
so disable _FORTIFY_SOURCE when debugging.
Signed-off-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

configure

@@ -256,7 +256,6 @@ QEMU_CFLAGS="-fno-strict-aliasing $QEMU_CFLAGS"
 QEMU_CFLAGS="-Wall -Wundef -Wwrite-strings -Wmissing-prototypes $QEMU_CFLAGS"
 QEMU_CFLAGS="-Wstrict-prototypes -Wredundant-decls $QEMU_CFLAGS"
 QEMU_CFLAGS="-D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE $QEMU_CFLAGS"
-QEMU_CFLAGS="-D_FORTIFY_SOURCE=2 $QEMU_CFLAGS"
 QEMU_INCLUDES="-I. -I\$(SRC_PATH) -I\$(SRC_PATH)/fpu"
 if test "$debug_info" = "yes"; then
     CFLAGS="-g $CFLAGS"
@@ -2856,7 +2855,7 @@ fi
 # After here, no more $cc or $ld runs
 
 if test "$debug" = "no" ; then
-  CFLAGS="-O2 $CFLAGS"
+  CFLAGS="-O2 -D_FORTIFY_SOURCE=2 $CFLAGS"
 fi
 
 # Consult white-list to determine whether to enable werror