backward-incompatible change in the 2008 POSIX specification.
- Add the .SHELLFLAGS variable so people can choose their own shell flags.
- Add tests for this.
- Add documentation for this.
follow POSIX backslash/newline conventions.
Use a different method for testing the SHELL variable, which hopefully
will work better on non-UNIX systems.
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.
