[gn build] Try harder to unbreak Windows build after f8e833a.

Author: nico

llvm/utils/gn/build/libs/zlib/BUILD.gn

@@ -2,9 +2,7 @@ import("//llvm/utils/gn/build/libs/zlib/enable.gni")
 
 config("zlib_config") {
   visibility = [ ":zlib" ]
-  if (host_os == "win") {
-    assert(!llvm_enable_zlib || zlib_path != "",
-           "Please provide the path to zlib.")
+  if (host_os == "win" && zlib_path != "") {
     include_dirs = [ zlib_path ]
     libs = [ "$zlib_path/zlib.lib" ]
   } else {
@@ -14,6 +12,8 @@ config("zlib_config") {
 
 group("zlib") {
   if (llvm_enable_zlib) {
+    assert(host_os != "win" || zlib_path != "",
+           "Please set zlib_path on Windows.")
     public_configs = [ ":zlib_config" ]
   }
 }