patch 8.1.2316: FORTIFY_SOURCE can also be present in CPPFLAGS

Problem:    FORTIFY_SOURCE can also be present in CPPFLAGS.
Solution:   Remove it in configure. (Benedikt Morbach, closes #2786)
diff --git a/src/auto/configure b/src/auto/configure
index 61b7272..d591ab5 100755
--- a/src/auto/configure
+++ b/src/auto/configure
@@ -12989,8 +12989,8 @@
 $as_echo "yes" >&6; }; $as_echo "#define HAVE_CANBERRA 1" >>confdefs.h
 
 else
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
-$as_echo "no" >&6; }; CFLAGS="$ac_save_CFLAGS"; LIBS="$ac_save_LIBS"
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no; try installing libcanberra-dev" >&5
+$as_echo "no; try installing libcanberra-dev" >&6; }; CFLAGS="$ac_save_CFLAGS"; LIBS="$ac_save_LIBS"
 fi
 rm -f core conftest.err conftest.$ac_objext \
     conftest$ac_exeext conftest.$ac_ext
@@ -14807,6 +14807,7 @@
 $as_echo_n "checking whether we need -D_FORTIFY_SOURCE=1... " >&6; }
   if test "$gccmajor" -gt "3"; then
     CFLAGS=`echo "$CFLAGS" | sed -e 's/ *-Wp,-D_FORTIFY_SOURCE=.//g' -e 's/ *-D_FORTIFY_SOURCE=.//g' -e 's/ *-U_FORTIFY_SOURCE//g' -e 's/$/ -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1/'`
+    CPPFLAGS=`echo "$CPPFLAGS" | sed -e 's/ *-Wp,-D_FORTIFY_SOURCE=.//g' -e 's/ *-D_FORTIFY_SOURCE=.//g' -e 's/ *-U_FORTIFY_SOURCE//g'`
     { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
 $as_echo "yes" >&6; }
   else