Firefox build fails on NetBSD_6.1-amd64

[Yoann Toussaint <yoann.tous1%gmail.com@localhost>]

I just switched to pkgsrc-2016Q2, and firefox 47.0.1nb1 fails on
NetBSD6.1_amd64 with the following error:

In file included from
/usr/pkgsrc/www/firefox/work/firefox-47.0/gfx/skia/skia/src/core/SkRRect.cpp:11:0,
from /usr/pkgsrc/www/firefox/work/build/gfx/skia/Unified_cpp_gfx_skia6.cpp:110:
/usr/pkgsrc/www/firefox/work/firefox-47.0/gfx/skia/skia/src/core/SkScaleToSides.h:
In static member function 'static void
ScaleToSides::AdjustRadii(double, double, SkScalar*, SkScalar*)':
/usr/pkgsrc/www/firefox/work/firefox-47.0/gfx/skia/skia/src/core/SkScaleToSides.h:52:61:
error: 'nexttowardf' was not declared in this scope
newMaxRadius = nexttowardf(newMaxRadius, 0.0);
^
gmake[3]: *** [/usr/pkgsrc/www/firefox/work/firefox-47.0/config/rules.mk:967:
Unified_cpp_gfx_skia6.o] Error 1
gmake[3]: Leaving directory '/usr/pkgsrc/www/firefox/work/build/gfx/skia'