isnan function conflicts with C++ standard library declarations

["Martin Storsjö" <martin@martin.st>]

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Hi,

When the isnan function is enabled in gnulib, the added bits in math.h 
break if included in C++ translation units.

A minimal reproducion example is available at 
https://martin.st/temp/gnulib-isnan-repro-0.0.0.tar.gz (preconfigured and 
directly buildable) and 
https://martin.st/temp/gnulib-isnan-repro-source.tar.gz (original source, 
requires running gnulib-tool and autoreconf).

The error manifests both with GCC/libstdc++ and Clang/libc++, with error 
messages like these:

In file included from myprog.cpp:2:0:
lib/math.h: In function ‘int isnan(float)’:
lib/math.h:2829:1: error: ‘int isnan(float)’ conflicts with a previous declaration
  _GL_MATH_CXX_REAL_FLOATING_DECL_2 (isnan)
  ^
In file included from /usr/include/c++/7/math.h:36:0,
                  from lib/math.h:27,
                  from myprog.cpp:2:
/usr/include/c++/7/cmath:618:3: note: previous declaration ‘constexpr bool std::isnan(float)’
    isnan(float __x)
    ^~~~~

Or this:

In file included from myprog.cpp:2:
lib/math.h:2829:36: error: 'isnan' is missing exception specification 'throw()'
_GL_MATH_CXX_REAL_FLOATING_DECL_2 (isnan)
                                    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/math.h:533:1: note:
       previous declaration is here
isnan(float __lcpp_x) _NOEXCEPT { return __libcpp_isnan(__lcpp_x); }
^

This issue can crop up e.g. when trying to build gettext for mingw (which 
has got the gnulib isnan function bundled, even if it isn't used).

// Martin