AC_CONFIG_FILES creates broken config.status when shell variable expansion is used

[Dustin Boyd]

AC_PREREQ([2.69])
AC_INIT([hello], [0.1])
AS_VAR_SET([hello_major], [0])
AC_CONFIG_FILES([hello-${hello_major}.pc:hello.pc.in])
AC_OUTPUT

This results in a config.status containing the following line:

config_files="hello-0.pc:hello.pc.in"

Without arguments, config.status works fine because it uses that line
to generate all items listed. If passed the argument hello-0.pc,
however, an "invalid argument" error results:

config.status: error: invalid argument: `hello-0.pc'

>From what I could find, the issue is in the "Handling of arguments"
section that appears later in the file:

"hello-${hello_major}.pc") CONFIG_FILES="$CONFIG_FILES
hello-${hello_major}.pc:hello.pc.in" ;;

It seems like the shell variable was expanded properly in the first
case for creating the config_files line, but this expansion did not
occur when creating the argument-handling section.

This is particularly problematic when combined with something like
Automake that automatically creates rules to regenerate the output
file when the template is modified.

This happens both with in-tree and out-of-source builds and can be
mitigated using an m4 macro:

m4_define([hello_major], [0])
AC_CONFIG_FILES([hello-]hello_major[.pc:hello.pc.in])