Comparing M4 to the C Preprocessor
M4 macros are similar in many ways to the C-preprocessor (CPP) macros defined in C-language source files. The C preprocessor is also a text replacement tool, which isn't surprising: Both M4 and the C preprocessor were designed and written by Kernighan and Ritchie around the same time.
Autoconf uses square brackets around macro parameters as a quoting mechanism. Quotes are necessary only for cases in which the context of the macro call could cause an ambiguity that the macro processor may resolve incorrectly (usually without telling you). We'll discuss M4 quoting in much more detail in Chapter 10. For now, just use square brackets around every argument to ensure that the expected macro expansions are generated. ...
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