Cygwin and Mingw floating-point fixes.

Deal with silent-underflow errors in float4 for cygwin and mingw by
using our strtof() wrapper; deal with misrounding errors by adding
them to the resultmap. Some slight reorganization of declarations was
done to avoid duplicating material between cygwin.h and win32_port.h.

While here, remove from the resultmap all references to
float8-small-is-zero; inspection of cygwin output suggests it's no
longer required there, and the freebsd/netbsd/openbsd entries should
no longer be necessary (these date back to c. 2000). This commit
doesn't remove the file itself nor the documentation references for
it; that will happen in a subsequent commit if all goes well.

configure

@@ -15828,6 +15828,25 @@ fi
 
 
 
+case $host_os in
+	# Cygwin and (apparently, based on test results) Mingw both
+	# have a broken strtof(), so substitute the same replacement
+	# code we use with VS2013. That's not a perfect fix, since
+	# (unlike with VS2013) it doesn't avoid double-rounding, but
+	# we have no better options. To get that, though, we have to
+	# force the file to be compiled despite HAVE_STRTOF.
+	mingw*|cygwin*)
+		case " $LIBOBJS " in
+  *" strtof.$ac_objext "* ) ;;
+  *) LIBOBJS="$LIBOBJS strtof.$ac_objext"
+ ;;
+esac
+
+		{ $as_echo "$as_me:${as_lineno-$LINENO}: On $host_os we will use our strtof wrapper." >&5
+$as_echo "$as_me: On $host_os we will use our strtof wrapper." >&6;}
+	;;
+esac
+
 case $host_os in
 
         # Windows uses a specialised env handler