diff --git a/m4/cares-compilers.m4 b/m4/cares-compilers.m4
index 156ff63..10a89e7 100644
--- a/m4/cares-compilers.m4
+++ b/m4/cares-compilers.m4
@@ -380,7 +380,7 @@ dnl not reliable on ancient GNUC versions.
 AC_DEFUN([CARES_CONVERT_INCLUDE_TO_ISYSTEM], [
   AC_REQUIRE([CARES_SHFUNC_SQUEEZE])dnl
   tmp_has_include="no"
-  tmp_chg_FLAGS=$CFLAGS
+  tmp_chg_FLAGS="$CFLAGS"
   for word1 in $tmp_chg_FLAGS; do
     case "$word1" in
       -I*)
@@ -389,13 +389,13 @@ AC_DEFUN([CARES_CONVERT_INCLUDE_TO_ISYSTEM], [
     esac
   done
   if test "$tmp_has_include" = "yes"; then
-    tmp_chg_FLAGS=`echo $tmp_chg_FLAGS | sed 's/^-I/ -isystem /g'`
-    tmp_chg_FLAGS=`echo $tmp_chg_FLAGS | sed 's/ -I/ -isystem /g'`
+    tmp_chg_FLAGS=`echo "$tmp_chg_FLAGS" | "$SED" 's/^-I/ -isystem /g'`
+    tmp_chg_FLAGS=`echo "$tmp_chg_FLAGS" | "$SED" 's/ -I/ -isystem /g'`
     CFLAGS="$tmp_chg_FLAGS"
     squeeze CFLAGS
   fi
   tmp_has_include="no"
-  tmp_chg_FLAGS=$CPPFLAGS
+  tmp_chg_FLAGS="$CPPFLAGS"
   for word1 in $tmp_chg_FLAGS; do
     case "$word1" in
       -I*)
@@ -404,8 +404,8 @@ AC_DEFUN([CARES_CONVERT_INCLUDE_TO_ISYSTEM], [
     esac
   done
   if test "$tmp_has_include" = "yes"; then
-    tmp_chg_FLAGS=`echo $tmp_chg_FLAGS | sed 's/^-I/ -isystem /g'`
-    tmp_chg_FLAGS=`echo $tmp_chg_FLAGS | sed 's/ -I/ -isystem /g'`
+    tmp_chg_FLAGS=`echo "$tmp_chg_FLAGS" | "$SED" 's/^-I/ -isystem /g'`
+    tmp_chg_FLAGS=`echo "$tmp_chg_FLAGS" | "$SED" 's/ -I/ -isystem /g'`
     CPPFLAGS="$tmp_chg_FLAGS"
     squeeze CPPFLAGS
   fi