configure: Recognize uname "Windows_NT" as using an .exe suffix

Busybox-w32 [1] works for building ffmpeg on Windows (as an
alternative to msys2, cygwin or WSL).

On busybox-w32, "uname" returns "Windows_NT"; recognize this
in exesuf() as having an .exe suffix.

If building in this environment with a mingw toolchain, one has
to explicitly set --target-os=mingw32. (We probably don't
want to imply that this uname, set as target_os_default, would
default to mingw?) But despite what is set with --target-os,
one can't override the configure variable "host_os", which
exesuf() has to recognize.

[1] https://github.com/rmyorston/busybox-w32

configure

@@ -4909,7 +4909,7 @@ fi
 
 exesuf() {
     case $1 in
-        mingw32*|mingw64*|msys*|win32|win64|cygwin*|*-dos|freedos|opendos|os/2*|symbian) echo .exe ;;
+        mingw32*|mingw64*|msys*|win32|win64|cygwin*|*-dos|freedos|opendos|os/2*|symbian|windows_nt) echo .exe ;;
     esac
 }