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   AC_CXX_COMPLEX_MATH_IN_NAMESPACE_STD
  
If the C math functions are in the cmath header file and std:: namespace, define HAVE_MATH_FN_IN_NAMESPACE_STD.
http://autoconf-archive.cryp.to/ac_cxx_complex_math_in_namespace_std.m4
    Copyright © 2008 Todd Veldhuizen
    Copyright © 2008 Luc Maisonobe <luc@spaceroots.org>
   
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