configure: call flatten_extralibs in a subshell

configure

@@ -6501,6 +6501,11 @@ flatten_extralibs(){
     fi
 }
 
+flatten_extralibs_echo(){
+    flatten_extralibs $1
+    eval echo \$$1
+}
+
 for linkunit in $LIBRARY_LIST; do
     unset current_extralibs
     eval components=\$$(toupper ${linkunit})_COMPONENTS_LIST
@@ -6513,7 +6518,7 @@ for linkunit in $LIBRARY_LIST; do
 done
 
 for linkunit in $LIBRARY_LIST $PROGRAM_LIST $EXTRALIBS_LIST; do
-    flatten_extralibs ${linkunit}_extralibs
+    eval ${linkunit}_extralibs=\$\(flatten_extralibs_echo ${linkunit}_extralibs\)
     unique  ${linkunit}_extralibs
     resolve ${linkunit}_extralibs
     eval ${linkunit}_extralibs=\$\(\$ldflags_filter \$${linkunit}_extralibs\)