configure: Avoid use of nonstandard features of sed

Standard sed does not support EREs.

Fixes #7310.

configure

@@ -3725,8 +3725,7 @@ find_things_extern(){
 
 find_filters_extern(){
     file=$source_path/$1
-    #sed -n "s/^extern AVFilter ff_\([avfsinkrc]\{2,5\}\)_\(\w\+\);/\2_filter/p" $file
-    sed -E -n "s/^extern AVFilter ff_([avfsinkrc]{2,5})_([a-zA-Z0-9_]+);/\2_filter/p" $file
+    sed -n 's/^extern AVFilter ff_[avfsinkrc]\{2,5\}_\([[:alnum:]_]\{1,\}\);/\1_filter/p' $file
 }
 
 FILTER_LIST=$(find_filters_extern libavfilter/allfilters.c)