configure script: --enable-unregisterised default printed incorrectly

The help text for the --enable-unregisterised flag to configure renders incorrectly. Instead of printing the actual default, it instead prints the name of the variable holding the default:

  --enable-unregisterised Build an unregisterised compiler (enabled by default
                          on platforms without registerised support)
                          [default="$UnregisterisedDefault"]

This happens because the help text is generated via a call to the AC_HELP_STRING macro:

AC_HELP_STRING([--enable-unregisterised],
[Build an unregisterised compiler (enabled by default on platforms without registerised support) [default="$UnregisterisedDefault"]])

According to the autoconf documentation, "the second argument of AS_HELP_STRING is treated as a whitespace separated list of text to be reformatted, and is not subject to macro expansion." (https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/html_node/Pretty-Help-Strings.html) - hence why the name of the variable is included in the output instead of its value.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Build System
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