gtk/gtklevelbar.c: Fix build on non-C99 compilers
gtk/gtklevelbar.c: Fix build on non-C99 compilers

-Include fallback-c89.c for the usage of round(), where an implementation
 of round() is provided for compilers that don't have it
-Use g_ascii_strtod() instead of strtof as strtof() may not be universally
 available.

alexlarsson/gtk,chergert/gtk,Distrotech/gtk2,davidt/gtk,msteinert/gtk,davidt/gtk,grubersjoe/adwaita,ahodesuka/gtk,ahodesuka/gtk,Lyude/gtk-,davidgumberg/gtk,davidgumberg/gtk,ebassi/gtk,chergert/gtk,jigpu/gtk,Adamovskiy/gtk,Adamovskiy/gtk,ahodesuka/gtk,ahodesuka/gtk,alexlarsson/gtk,alexlarsson/gtk,jigpu/gtk,ebassi/gtk,bratsche/gtk-,Sidnioulz/SandboxGtk,Lyude/gtk-,jadahl/gtk,jessevdk/gtk,chergert/gtk,msteinert/gtk,grubersjoe/adwaita,chergert/gtk,alexlarsson/gtk,jessevdk/gtk,ebassi/gtk,grubersjoe/adwaita,msteinert/gtk,jigpu/gtk,jigpu/gtk,jigpu/gtk,grubersjoe/adwaita,Lyude/gtk-,bratsche/gtk-,davidt/gtk,jadahl/gtk,davidgumberg/gtk,Adamovskiy/gtk,Lyude/gtk-,davidgumberg/gtk,jessevdk/gtk,grubersjoe/adwaita,davidgumberg/gtk,ahodesuka/gtk,msteinert/gtk,Lyude/gtk-,davidgumberg/gtk,jessevdk/gtk,jadahl/gtk,Adamovskiy/gtk,jigpu/gtk,ebassi/gtk,jessevdk/gtk,bratsche/gtk-,msteinert/gtk,davidt/gtk,alexlarsson/gtk,jessevdk/gtk,jadahl/gtk,grubersjoe/adwaita,jadahl/gtk,jadahl/gtk,jadahl/gtk,Distrotech/gtk2,davidt/gtk,jessevdk/gtk,alexlarsson/gtk,Distrotech/gtk2,Sidnioulz/SandboxGtk,Distrotech/gtk2,grubersjoe/adwaita,Distrotech/gtk2,ebassi/gtk,ahodesuka/gtk,bratsche/gtk-,ahodesuka/gtk,Sidnioulz/SandboxGtk,alexlarsson/gtk,grubersjoe/adwaita,Adamovskiy/gtk,msteinert/gtk,Adamovskiy/gtk,ebassi/gtk,bratsche/gtk-,Sidnioulz/SandboxGtk,chergert/gtk,Sidnioulz/SandboxGtk,davidgumberg/gtk,jadahl/gtk,Adamovskiy/gtk,jigpu/gtk,chergert/gtk,Sidnioulz/SandboxGtk,Lyude/gtk-,Distrotech/gtk2,jigpu/gtk,ahodesuka/gtk,alexlarsson/gtk,Adamovskiy/gtk,Lyude/gtk-,davidgumberg/gtk,Lyude/gtk-,bratsche/gtk-,davidt/gtk,chergert/gtk,chergert/gtk