build errors: "type of ‘std::copysign’ is unknown"

[Colin Macdonald]

I updated my system to Fedora 24; when rebuilding Octave dev I get the 
following errors (any many more for other math.h functions).

Anyone recognize this?

-----------------------------------------------------

  CXX      libgui/src/m-editor/libgui_src_libgui_src_la-octave-txt-lexer.lo
In file included from /usr/include/sys/types.h:219:0,
                 from libgnu/sys/types.h:28,
                 from /usr/include/stdlib.h:314,
                 from /usr/include/c++/6.1.1/cstdlib:75,
                 from /usr/include/c++/6.1.1/bits/stl_algo.h:59,
                 from /usr/include/c++/6.1.1/algorithm:62,
                 from /usr/include/QtCore/qglobal.h:68,
                 from /usr/include/QtCore/qnamespace.h:45,
                 from /usr/include/QtCore/qobjectdefs.h:45,
                 from /usr/include/QtCore/qobject.h:47,
                 from /usr/include/QtCore/qabstracteventdispatcher.h:45,
                 from /usr/include/QtCore/QtCore:3,
                 from /usr/include/QtGui/QtGui:3,
                 from libgui/src/m-editor/find-dialog.cc:68:
libgnu/math.h:775:1: error: type of ‘std::copysign’ is unknown
 _GL_CXXALIASWARN (copysign);
 ^
libgnu/math.h:775:1: error: ‘int copysign’ redeclared as different kind 
of symbol
 _GL_CXXALIASWARN (copysign);
 ^
In file included from /usr/include/c++/6.1.1/math.h:36:0,
                 from libgnu/math.h:27,
                 from /usr/include/QtCore/qmath.h:45,
                 from /usr/include/QtCore/QtCore:9,
                 from /usr/include/QtGui/QtGui:3,
                 from libgui/src/m-editor/find-dialog.cc:68:
/usr/include/c++/6.1.1/cmath:1288:3: note: previous declaration 
‘constexpr float std::copysign(float, float)’
   copysign(float __x, float __y)
   ^~~~~~~~

-----------------------------------------------------

Previously to this error, I was getting:

libinterp/corefcn/oct-tex-parser.cc: In function ‘int 
octave_tex_parse(text_parser_tex&)’:
./oct-conf-post.h:24:28: error: ‘malloc’ is not a member of ‘gnulib’
 #  define GNULIB_NAMESPACE gnulib

I installed a `gnulib-devel` that I don't recall needed on Fedora 23 
which got paste that part but perhaps causes the above?

thanks,
Colin