[SV 63040] shell: Fall back to the callers environment

If we detect a recursive variable reference when constructing the
environment for the shell function, return the original value from the
caller's environment.  Other options such as failing, returning the
empty string, or returning the unexpanded make variable value have
been shown to not behave well in real-world environments.  If the
variable doesn't exist in the caller's environment, return the empty
string.

Found by Sergei Trofimovich <slyich@gmail.com> when testing older
versions of autoconf.

* NEWS: Clarify this behavior.
* doc/make.texi (Shell Function): Ditto.  Also add info about !=.
* src/expand.c (recursively_expand_for_file): Search the caller's
environment if we detect a recursive variable expansion.
* tests/scripts/functions/shell: Add tests for this behavior.

tests/scripts/functions/shell

@@ -98,6 +98,22 @@ all:; : $(HI)
 ',
               '',": hi");
 
+$ENV{HI} = 'outer';
+run_make_test('
+export HI = $(shell echo $$HI)
+.PHONY: all
+all:; @echo $$HI
+',
+              '',"outer");
+
+$ENV{HI} = 'outer';
+run_make_test('
+export HI = $(shell echo $$HI)
+.PHONY: all
+all:; : $(HI)
+',
+              '',": outer");
+
 if ($port_type ne 'W32') {
     # Test shell errors in recipes including offset
     # This needs to be ported to Windows, or else Windows error messages