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

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.