makefiles: use bash subshell

Avoid using  make's $(shell invocation since the eval order is
then somewhat different and use $$(  subshell.

This also fixes a problem when more then one symbol is found,
since target shell has been given separate word list
so the 'R' assignment would need "" around it.

make.tmpl.in

@@ -499,7 +499,7 @@ ifeq (,$(firstword $(EXPORTED_SYMBOLS)))
 		 echo "	local:"; echo "		*;"; echo "};") > $@
 else
 	set -e;\
-	R=$(shell sort $^ | uniq -u);\
+	R=$$(sort $^ | uniq -u);\
 	test -z "$$R" || { echo "Mismatch between symbols in shared library and lists in .exported_symbols.* files: $$R"; false; } ;\
 	for i in $(EXPORTED_SYMBOLS); do\
 		echo "$${i##*.} {"; echo "	global:";\