Labplot master branch build failures

[Rik Mills]

Seems to fail with GCC 6, but succeeds with GCC 5.

Failures on GCC6:

artful http://kci.pangea.pub/job/artful_unstable_labplot/45/
zesty http://kci.pangea.pub/job/zesty_unstable_labplot/52/

make[3]: *** [src/CMakeFiles/labplot2.dir/kdefrontend/GuiObserver.cpp.o]
Error 1
make[3]: *** Waiting for unfinished jobs....
In file included from
/<<BUILDDIR>>/labplot-2.4.0+p17.10+git20170613.0754/src/backend/spreadsheet/Spreadsheet.h:33:0,
                 from
/<<BUILDDIR>>/labplot-2.4.0+p17.10+git20170613.0754/src/kdefrontend/MainWin.cpp:36:
/<<BUILDDIR>>/labplot-2.4.0+p17.10+git20170613.0754/src/backend/core/column/Column.h:
In constructor ‘Column::ColumnStatistics::ColumnStatistics()’:
/<<BUILDDIR>>/labplot-2.4.0+p17.10+git20170613.0754/src/backend/core/column/Column.h:46:15:
error: ‘NAN’ was not declared in this scope
     minimum = NAN;
               ^~~
src/CMakeFiles/labplot2.dir/build.make:372: recipe for target
'src/CMakeFiles/labplot2.dir/kdefrontend/MainWin.cpp.o' failed

Success on GCC 5:

xenial KCI http://kci.pangea.pub/job/xenial_unstable_labplot/55/

and for ref on other CI using GCC5

xenial NCI
http://build.neon.kde.org/job/xenial_unstable_kde-extras_labplot/41/
KDE CI https://build.kde.org/job/labplot%20master%20kf5-qt5/180/