Some compilers we support, such as pre-2013 Visual Studio, does not support
for INIFINITY, log2() and exp2(), so check for exp2() and log2() during
configure, and use fallbacks for them and INIFINTY if they are not found.
https://bugzilla.gnome.org/show_bug.cgi?id=766207
... on older Visual Studio versions, where isinf() is not available, and
copy the isinf() implementation from gdk/fallback-c89.c to
gtk/fallback-c89.c.
Add an implementation for rint() and round() for compilers that do not
define one or both of them. Note that this file should be included
after config.h was included.
