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.