The missing script is a wrapper around several maintainer tools, designed to warn users if a maintainer tool is required but missing. Typical maintainer tools are autoconf, automake, bison, etc. Because file generated by these tools are shipped with the other sources of a package, these tools shouldn't be required during a user build and they are not checked for in configure.
However, if for some reason a rebuild rule is triggered and involves a missing tool, missing will notice it and warn the user, even suggesting how to obtain such a tool (at least in case it is a well-known one, like makeinfo or bison). This is more helpful and user-friendly than just having the rebuild rules spewing out a terse error message like ‘sh: tool: command not found’. Similarly, missing will warn the user if it detects that a maintainer tool it attempted to use seems too old (be warned that diagnosing this correctly is typically more difficult that detecting missing tools, and requires cooperation from the tool itself, so it won't always work).
If the required tool is installed, missing will run it and
won't attempt to continue after failures. This is correct during
development: developers love fixing failures. However, users with
missing or too old maintainer tools may get an error when the rebuild
rule is spuriously triggered, halting the build. This failure to let
the build continue is one of the arguments of the
AM_MAINTAINER_MODE allows you to choose whether the so called
"rebuild rules" should be enabled or disabled. With
AM_MAINTAINER_MODE([enable]), they are enabled by default,
otherwise they are disabled by default. In the latter case, if
AM_MAINTAINER_MODE in configure.ac, and run
‘./configure && make’, then make will *never* attempt to
rebuild configure, Makefile.ins, Lex or Yacc outputs, etc.
I.e., this disables build rules for files that are usually distributed
and that users should normally not have to update.
The user can override the default setting by passing either ‘--enable-maintainer-mode’ or ‘--disable-maintainer-mode’ to configure.
AM_MAINTAINER_MODE either because they do not want their
users (or themselves) annoyed by timestamps lossage (see CVS), or
because they simply can't stand the rebuild rules and prefer running
maintainer tools explicitly.
AM_MAINTAINER_MODE also allows you to disable some custom build
rules conditionally. Some developers use this feature to disable
rules that need exotic tools that users may not have available.
Several years ago François Pinard pointed out several arguments
AM_MAINTAINER_MODE macro. Most of them relate to
insecurity. By removing dependencies you get non-dependable builds:
changes to sources files can have no effect on generated files and this
can be very confusing when unnoticed. He adds that security shouldn't
be reserved to maintainers (what --enable-maintainer-mode
suggests), on the contrary. If one user has to modify a
Makefile.am, then either Makefile.in should be updated
or a warning should be output (this is what Automake uses
missing for) but the last thing you want is that nothing
happens and the user doesn't notice it (this is what happens when
rebuild rules are disabled by
Jim Meyering, the inventor of the
AM_MAINTAINER_MODE macro was
swayed by François's arguments, and got rid of
AM_MAINTAINER_MODE in all of his packages.
Still many people continue to use
it helps them working on projects where all files are kept under version
control, and because missing isn't enough if you have the
wrong version of the tools.