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   AC_CXX_EXPLICIT
  
If the compiler can be asked to prevent using implicitly one argument constructors as converting constructors with the explicit keyword, define HAVE_EXPLICIT.
http://autoconf-archive.cryp.to/ac_cxx_explicit.m4
    Copyright © 2008 Todd Veldhuizen
    Copyright © 2008 Luc Maisonobe <luc@spaceroots.org>
   
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