cmake: fix syntax error in Win32Deps.cmake

In a construct such as:

```cmake
if($ENV{FOO} MATCHES bar)
...
endif()
```

if the environment variable is not set, the if statement becomes a
syntax error because there is nothing being compared.

Fix this by quoting the environment variable like so:

```cmake
if("$ENV{FOO}" MATCHES bar)
...
endif()
```

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

cmake/Win32Deps.cmake

@@ -5,7 +5,7 @@ if(WIN32)
     endif()
 
     set(WINARCH x86)
-    if(CMAKE_C_COMPILER MATCHES x64 OR CMAKE_CXX_COMPILER MATCHES x64 OR $ENV{VSCMD_ARG_TGT_ARCH} MATCHES x64)
+    if(CMAKE_C_COMPILER MATCHES x64 OR CMAKE_CXX_COMPILER MATCHES x64 OR "$ENV{VSCMD_ARG_TGT_ARCH}" MATCHES x64)
         set(WINARCH x64)
     endif()