Autoconf, Automake, and Libtool: Primitive macros to aid in building macros

| ### 23.5.2 Primitive macros to aid in building macros Writing new macros is one aspect of Autoconf that has proven troublesome to users in the past, since this is one area where Autoconf's implementation details leak out. Autoconf extensively uses `m4` to perform the translation of ``configure.in'` to ``configure'`. Thus, it is necessary to understand implementation details such as M4s quoting rules in order to write Autoconf macros (21. M4). Another aspect of macro writing which is extremely hard to get right is writing portable Bourne shell scripts (see section 22. Writing Portable Bourne Shell). Writing portable software, be it in Bourne shell or C++, is something that can only be mastered with years of experience--and exposure to many different kinds of machines! Rather than expect all macro writers to acquire this experience, it makes sense for Autoconf to become a `knowledge base' for this experience. With this in mind, one future direction for Autoconf will be to provide a library of low-level macros to assist in writing new macros. By way of hypothetical example, consider the benefit of using a macro named `AC_FOREACH` instead of needing to learn the hard way that some vendor's implementation of Bourne shell has a broken `for` loop construct. This idea will be explored in future versions of Autoconf. When migrating existing packages to the GNU Autotools, which is the topic of the next chapter, it is worth remember these guidelines for best practices as you write the necessary tests to make those packages portable.

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