AS_IF: Handle else clause being empty after macro expansion (#110369)

AS_IF: Handle else clause being empty after macro expansion (#110369)

AS_IF can emit a syntactically invalid shell if-then-else,

  if CONDITION
  then :
    # ...
  else
  fi

when its IF-FALSE argument consists of macros that don’t produce any
shell code.  This was a documented limitation in AS_IF, but it’s a bad
limitation to have, because macros that *used* to expand to shell
commands might start expanding to nothing in future releases.  For
instance, this broke the libzmq configure script, which did

  AC_PROG_CC
  AX_CHECK_COMPILE_FLAG([-std=gnu11],
    [CFLAGS+=" -std=gnu11"],
    [AC_PROG_CC_C99])

Perfectly valid in 2.69, but in 2.70 AC_PROG_CC_C99 doesn’t produce
any shell code and the script crashes.

We had that limitation for good reason: we can’t just put ‘:’ at the
beginning of the else-clause, like we do for the then-clause, because
that would clobber $? and the IF-FALSE commands might want to inspect
it.  (This doesn’t matter for the then-clause, because $? is always
zero at the beginning of a then-clause anyway.)  The simplest and
least inefficient shell construct I can find that works in this
context is a shell function that does ‘return $?’.  Due to awkward
M4sh initialization ordering constraints (AS_IF gets used before we
can safely use shell functions) an indirection through a shell
variable is necessary.  The structure of a m4sh script is now

  #! /bin/sh
  ## M4sh Initialization
  as_nop=:

  ...
  ## M4sh Shell Functions

  as_fn_nop () { return $?; }
  as_nop=as_fn_nop
  ...

and AS_IF emits

  if CONDITION
  then :
    # ...
  else $as_nop
    # ...
  fi

The uses of AS_IF that appear before the beginning of the M4sh Shell
Functions section are all under our control and they don’t need to
look at $?.

If anyone has a better idea for how to make this work I will be glad
to hear it.

Fixes bug #110369.

* lib/m4sugar/m4sh.m4
  (_AS_IF_ELSE): When $1 is nonempty, invoke _AS_EMPTY_ELSE_PREPARE.
  Emit $as_nop at beginning of else clause.
  (_AS_BOURNE_COMPATIBLE): Initialize as_nop to ‘:’.
  (_AS_EMPTY_ELSE_PREPARE): New macro which emits a definition of
  as_fn_nop and resets as_nop to as_fn_nop.
  (AS_PREPARE, _AS_PREPARE): Invoke _AS_EMPTY_ELSE_PREPARE.
  (_AS_UNSET_PREPARE): Tweak white space.

* tests/m4sh.at (AS_IF and AS_CASE): Test AS_IF’s IF-FALSE argument
  being empty after macro expansion.

* doc/autoconf.texi (AS_IF): Remove warning about use with
  ‘run-if-false’ argument empty after macro expansion.