configure: don't depend on GNU sed

Closes #3169.

configure

@@ -5657,8 +5657,11 @@ extract() {
     return 1
   fi
 
+  # BSD sed needs an actual newline character in the substitute command
+  new_line='
+'
   # Convert " -" to "\n" if not between quotes and remove possible leading white spaces
-  string=$(echo " $*" | $SED -e 's: -:\n:g' -e 's:"\(.*\)\n\(.*\)":\"\1 -\2":g' -e "s:'\(.*\)\n\(.*\)':\'\1 -\2':g" -e 's/^[:space:]*//')
+  string=$(echo " $*" | $SED -e "s: -:\\${new_line}:g" -e 's:"\(.*\)\n\(.*\)":\"\1 -\2":g' -e "s:'\(.*\)\n\(.*\)':\'\1 -\2':g" -e 's/^[:space:]*//')
   SAVEIFS=$IFS
   IFS=$(printf "\n\b")
   for i in $string; do

configure.ac

@@ -222,8 +222,11 @@ extract() {
     return 1
   fi
 
+  # BSD sed needs an actual newline character in the substitute command
+  new_line='
+'
   # Convert " -" to "\n" if not between quotes and remove possible leading white spaces
-  string=$(echo " $*" | $SED -e 's: -:\n:g' -e 's:"\(.*\)\n\(.*\)":\"\1 -\2":g' -e "s:'\(.*\)\n\(.*\)':\'\1 -\2':g" -e 's/^[[:space:]]*//')
+  string=$(echo " $*" | $SED -e "s: -:\\${new_line}:g" -e 's:"\(.*\)\n\(.*\)":\"\1 -\2":g' -e "s:'\(.*\)\n\(.*\)':\'\1 -\2':g" -e 's/^[[:space:]]*//')
   SAVEIFS=$IFS
   IFS=$(printf "\n\b")
   for i in $string; do