Fix for bug #1276: Handle SHELL according to POSIX requirements.

POSIX requires that the value of SHELL in the makefile NOT be exported
to sub-commands.  Instead, the value in the environment when make was
invoked should be passed to the environment of sub-commands.  Note that
make still uses SHELL to _run_ sub-commands; it just doesn't change the
value of the SHELL variable in the environment of sub-commands.

As an extension to POSIX, if the makefile explicitly exports SHELL then
GNU make _will_ use it in the environment of sub-commands.

tests/scripts/variables/MAKELEVEL

@@ -1,4 +1,4 @@
-#                                                               -*-perl-mode-*-
+#                                                                    -*-perl-*-
 
 $description = "The following test creates a makefile to test
 makelevels in Make. It prints \$(MAKELEVEL) and then
@@ -9,7 +9,6 @@ open(MAKEFILE,"> $makefile");
 # The Contents of the MAKEFILE ...
 
 print MAKEFILE <<EOF;
-SHELL = /bin/sh
 all:
 \t\@echo MAKELEVEL is \$(MAKELEVEL)
 \techo \$\$MAKELEVEL

tests/scripts/variables/SHELL

@@ -0,0 +1,49 @@
+#                                                                    -*-perl-*-
+
+$description = "Test proper handling of SHELL.";
+
+# Find the default value when SHELL is not set.  On UNIX it will be /bin/sh,
+# but on other platforms who knows?
+$oshell = $ENV{SHELL};
+delete $ENV{SHELL};
+$mshell = `echo 'all:;\@echo \$(SHELL)' | $make_name -f-`;
+chop $mshell;
+
+# According to POSIX, the value of SHELL in the environment has no impact on
+# the value in the makefile.
+
+$ENV{SHELL} = '/dev/null';
+run_make_test('all:;@echo "$(SHELL)"', '', $mshell);
+
+# According to POSIX, any value of SHELL set in the makefile should _NOT_ be
+# exported to the subshell!  I wanted to set SHELL to be $^X (perl) in the
+# makefile, but make runs $(SHELL) -c 'commandline' and that doesn't work at
+# all when $(SHELL) is perl :-/.  So, we just add an extra initial / and hope
+# for the best on non-UNIX platforms :-/.
+
+$ENV{SHELL} = $mshell;
+
+run_make_test("SHELL := /$mshell\n".'
+all:;@echo "$(SHELL) $$SHELL"
+', '', "/$mshell $mshell");
+
+# As a GNU make extension, if make's SHELL variable is explicitly exported,
+# then we really _DO_ export it.
+
+run_make_test("export SHELL := /$mshell\n".'
+all:;@echo "$(SHELL) $$SHELL"
+', '', "/$mshell /$mshell");
+
+
+# Test out setting of SHELL, both exported and not, as a target-specific
+# variable.
+
+run_make_test("all: SHELL := /$mshell\n".'
+all:;@echo "$(SHELL) $$SHELL"
+', '', "/$mshell $mshell");
+
+run_make_test("all: export SHELL := /$mshell\n".'
+all:;@echo "$(SHELL) $$SHELL"
+', '', "/$mshell $mshell");
+
+1;