win: has("wsl") on Windows Subsystem for Linux #7330

Per CMAKE docs, CMAKE_HOST_SYSTEM_VERSION is the result of `uname -r`:
https://cmake.org/cmake/help/v3.4/variable/CMAKE_HOST_SYSTEM_VERSION.html?highlight=uname

    A numeric version string for the system. On systems that support
    uname, this variable is set to the output of uname -r. On other
    systems this is set to major-minor version numbers.

On Windows it is something like "6.1", so it won't match ".*-Microsoft".

Closes #7329

test/functional/eval/has_spec.lua

@@ -57,4 +57,10 @@ describe('has()', function()
       eq(0, funcs.has("unnamedplus"))
     end
   end)
+
+  it('"wsl"', function()
+    if 1 == funcs.has('win32') or 1 == funcs.has('mac') then
+      eq(0, funcs.has('wsl'))
+    end
+  end)
 end)