Fix qmake build

... when no llvm-config is found.
We must only error out if the user explicitly provided a path and it
didn't work.
Amends 3e2d224722.

Change-Id: I6f81f36467571e1e05a9bab9420ee59caa9965a2
Reviewed-by: hjk <hjk@qt.io>

src/shared/clang/clang_installation.pri

@@ -14,8 +14,11 @@ isEmpty(LLVM_CONFIG) {
 
 defineReplace(llvmWarningOrError) {
     warningText = $$1
-    isEmpty(llvm_config): warning($$warningText)
-    else: error($$warningText)
+    isEmpty(LLVM_CONFIG):isEmpty(LLVM_INSTALL_DIR) {
+        warning($$warningText)
+    } else {
+        error($$warningText)
+    }
     return(false)
 }