Use ISNAN() to prevent build error on OS X 10.7

[peternowee]

Replace isnan() with R's ISNAN(), as described in Writing R Extensions:

Functions/macros such as isnan, isinf and isfinite are not required by
C++98: where compilers support them they may be only in the std
namespace or only in the main namespace. There is no way to make use
of these functions which works with all C++ compilers currently in use
on R platforms: use R’s versions such as ISNAN and R_FINITE instead.
-- R Core Team,Writing R Extensions: Portable C and C++ code

This prevents the following error when trying to build RMySQL on my
Mac OS X 10.7:

MyBinding.h:100: error: ‘isnan’ was not declared in this scope

Further reading:

• https://cran.r-project.org/doc/manuals/r-release/R-exts.html#Portable-C-and-C_002b_002b-code
• https://stat.ethz.ch/pipermail/r-devel/2005-January/thread.html#31793
• https://stat.ethz.ch/pipermail/r-devel/2005-January/thread.html#31800
• http://lists.apple.com/archives/Xcode-users/2006/Aug/thrd7.html#00619
• http://lists.apple.com/archives/Xcode-users/2006/Aug/thrd7.html#00633
• http://stackoverflow.com/questions/18128899/is-isnan-in-the-std-namespace-more-in-general-when-is-std-necessary-optio
• http://stackoverflow.com/questions/2249110/how-do-i-make-a-portable-isnan-isinf-function
• http://stackoverflow.com/questions/570669/checking-if-a-double-or-float-is-nan-in-c

[hadley]

Thanks!

[peternowee]

You're welcome and thank you too for all your great work!