maint: avoid spurious output if program not present

maint: avoid spurious output if program not present

Some shells warn about missing programs before redirection;
the idiomatic way to silence them is to run the program check
inside a subshell, with the redirections outside the subshell.
But a subshell is only needed in places where it is reasonable
to expect the use of such a noisy shell in the first place.

* src/Makefile.am (remote_protocol-structs): Use subshell, for
FreeBSD 8.0 /bin/sh.
* cfg.mk (sc_preprocessor_indentation): Avoid subshell, since the
only users running cfg.mk can be assumed to have decent tools.