Optimization bug when byte compiling with gcc 5.2.0 on windows

I tested this hypothesis by replacing '__GNUC__ <= 4' with '__GNUC__
<= 5' and rebuilding R, but this introduces a whole lot of problems
that were not there before. In particular R crashes when using the
graphics device, which it did not without this fix.

I found that the problem goes away when we use isnanl() instead of
isnan(). This is very similar to the problem we have for R_pow() where
we need to use powl() instead of pow() for recent versions mingw-w64
(this still needs to be patched in r-devel).

So the full solution is:

#if (defined(_WIN32) || defined(_WIN64)) && defined(__GNUC__)
# define R_sqrt(x) (isnanl(x) ? x : sqrt(x))
#else
# define R_sqrt sqrt
#endif

Below the relevant mingw-w64 sources:

https://github.com/Alexpux/mingw-w64/blob/master/mingw-w64-crt/math/isnan.c
https://github.com/Alexpux/mingw-w64/blob/master/mingw-w64-crt/math/isnanf.c
https://github.com/Alexpux/mingw-w64/blob/master/mingw-w64-crt/math/isnanl.c

I think mingw-w64 takes powl from glibc.