fmin() is a function that is introduced with C99/C++11, so check for the
presence of it and provide a simple implementation for it if it does not
exist.
Also update the config.h.win32.in template accordingly, since this
function is provided on Visual Studio 2013 or later.
This code made use of isnan(), which is not provided by pre-2013 Visual
Studio, so fix the build by including fallback-c89.c in place of math.h,
and update fallback-c89.c to include the implementation of isnan() for
Visual Studio, which is copied from the fallback-c89.c in gdk/.
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.
