build-sys: use _FORTIFY_SOURCE only if optimizing

Rather then force the user to undefine _FORTIFY_SOURCE,
don't define it in the first place if it cannot be used.

I'm assuming that -O* can only be sensibly specified in $CFLAGS.

autogen.sh

@@ -54,7 +54,7 @@ args="$args \
 fi
 
 if [ "x$1" = "xc" ]; then
-        ./configure CFLAGS='-g -O0 -Wp,-U_FORTIFY_SOURCE' $args
+        ./configure CFLAGS='-g -O0' $args
         make clean
 else
         echo
@@ -62,6 +62,6 @@ else
         echo "Initialized build system. For a common configuration please run:"
         echo "----------------------------------------------------------------"
         echo
-        echo "./configure CFLAGS='-g -O0 -Wp,-U_FORTIFY_SOURCE' $args"
+        echo "./configure CFLAGS='-g -O0' $args"
         echo
 fi

configure.ac

@@ -167,8 +167,10 @@ CC_CHECK_FLAGS_APPEND([with_cflags], [CFLAGS], [\
         --param=ssp-buffer-size=4])
 AC_SUBST([OUR_CFLAGS], $with_cflags)
 
-CC_CHECK_FLAGS_APPEND([with_cppflags], [CPPFLAGS], [\
-        -Wp,-D_FORTIFY_SOURCE=2])
+AS_CASE([$CFLAGS], [*-O[[12345\ ]]*], [
+        CC_CHECK_FLAGS_APPEND([with_cppflags], [CPPFLAGS], [\
+               -Wp,-D_FORTIFY_SOURCE=2])], [
+        AC_MSG_RESULT([skipping -D_FORTIFY_SOURCE, optimization not enabled])])
 AC_SUBST([OUR_CPPFLAGS], $with_cppflags)
 
 CC_CHECK_FLAGS_APPEND([with_ldflags], [LDFLAGS], [\