[c++tools] Fix typo and weird syntax in configure script

[Eric Botcazou]

Tested on x86-64/Linux, applied on mainline and 11 branch as obvious.


2021-05-25  Eric Botcazou  <ebotcazou@adacore.com>

c++tools/
	* configure.ac (--enable-maintainer-mode): Fix typo and weird syntax.
	* configure: Regenerate.

-- 
Eric Botcazou
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