Fix misdetecting 64-bit MinGW

`gcc -dumpmachine` returns `x86_64-w64-mingw32` on 64-bit MinGW-w64.

Makefile

@@ -2,13 +2,14 @@ ifeq ($(OS),Windows_NT)
     # Need to figure out if Cygwin/Mingw is installed
     SYS := $(shell gcc -dumpmachine)
     ifeq ($(findstring cygwin, $(SYS)),cygwin)
-      PLATFORM = cygwin
-    endif
-    ifeq ($(findstring mingw32, $(SYS)),mingw32)
-      PLATFORM = mingw32
-    endif
-    ifeq ($(findstring mingw64, $(SYS)),mingw64)
-      PLATFORM = mingw64
+        PLATFORM = cygwin
+    endif
+    ifeq ($(findstring mingw, $(SYS)),mingw)
+        ifeq ($(findstring x86_64, $(SYS)),x86_64)
+            PLATFORM = mingw64
+        else
+            PLATFORM = mingw32
+        endif
     endif
 else
     # Grab the output of `uname -s` and switch to set the platform

tools/appveyor.bat

@@ -71,7 +71,7 @@ bash -lc ""
 bash -lc "for i in {1..3}; do update-core && break || sleep 15; done"
 bash -lc "for i in {1..3}; do pacman --noconfirm -Su mingw-w64-%MSYS2_ARCH%-{gcc,libiconv} automake autoconf make dos2unix && break || sleep 15; done"
 
-bash -lc "make -f make_mingw%BIT%.mak"
+bash -lc "make"
 
 @echo off
 goto :eof