doc make "set -e" execution

[Kevin Ryde]

        * doc/autoconf.texi (Limitations of Make): Add notes on "set -e"
        execution by old makes.

This raised its ugly head in automake and in some stuff I was doing
recently.  I imagine it's a rather classic portability matter, though
I don't think I've had the pleasure before.

New words below, I'm not sure where to put them in the section, just
chucked at the end I suppose.

What's the procedure for checkins these days?  Is anyone giving
approvals, or for uncontroversial stuff can I just wait a polite
interval then go ahead?


`set -e'
     POSIX says that `make' runs commands as per the `system' function,
     which normally means `sh -c ...'.  Some old `make's (eg. NetBSD
     1.4) however run `sh -ec ...', which means a rule with a list of
     commands like `foo; bar; quux' will fail if any of the parts
     fails, not just the last.  (For more on `-e', *note The Set
     Builtin: (bash)The Set Builtin.)

     Rules can be written with something like `foo || true; ...' to
     ensure the result from each intermediate part is true.