Rethink how to get float.h in old Windows API for isnan/isinf

We include <float.h> in every place that needs isnan(), because MSVC
used to require it.  However, since MSVC 2013 that's no longer necessary
(cf. commit cec8394b5c), so we can retire the inclusion to a
version-specific stanza in win32_port.h, where it doesn't need to
pollute random .c files.  The header is of course still needed in a few
places for other reasons.

I (Álvaro) removed float.h from a few more files than in Emre's original
patch.  This doesn't break the build in my system, but we'll see what
the buildfarm has to say about it all.

Author: Emre Hasegeli
Discussion: https://postgr.es/m/CAE2gYzyc0+5uG+Cd9-BSL7NKC8LSHLNg1Aq2=8ubjnUwut4_iw@mail.gmail.com

src/include/port/win32_port.h

@@ -502,7 +502,14 @@ typedef unsigned short mode_t;
 #define W_OK 2
 #define R_OK 4
 
+/*
+ * isinf() and isnan() should per spec be in <math.h>, but MSVC older than
+ * 2013 does not have them there.  It does have _fpclass() and _isnan(), but
+ * they're in <float.h>, so include that here even though it means float.h
+ * percolates to our whole tree.  Recent versions don't require any of this.
+ */
 #if (_MSC_VER < 1800)
+#include <float.h>
 #define isinf(x) ((_fpclass(x) == _FPCLASS_PINF) || (_fpclass(x) == _FPCLASS_NINF))
 #define isnan(x) _isnan(x)
 #endif