automake-1.16: Canonicalization

 
 3.5 How derived variables are named
 ===================================
 
 Sometimes a Makefile variable name is derived from some text the
 maintainer supplies.  For instance, a program name listed in ‘_PROGRAMS’
 is rewritten into the name of a ‘_SOURCES’ variable.  In cases like
 this, Automake canonicalizes the text, so that program names and the
 like do not have to follow Makefile variable naming rules.  All
 characters in the name except for letters, numbers, the strudel (@), and
 the underscore are turned into underscores when making variable
 references.
 
    For example, if your program is named ‘sniff-glue’, the derived
 variable name would be ‘sniff_glue_SOURCES’, not ‘sniff-glue_SOURCES’.
 Similarly the sources for a library named ‘libmumble++.a’ should be
 listed in the ‘libmumble___a_SOURCES’ variable.
 
    The strudel is an addition, to make the use of Autoconf substitutions
 in variable names less obfuscating.