[CMake] Check if passthrough variables are defined

Checking if they evaluate to true cases prevents passing values that evaluate to false cases. Instead we should check if the variables are defined.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@282122 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -610,7 +610,7 @@ if (CLANG_ENABLE_BOOTSTRAP)
 
   # Populate the passthrough variables
   foreach(variableName ${CLANG_BOOTSTRAP_PASSTHROUGH} ${_BOOTSTRAP_DEFAULT_PASSTHROUGH})
-    if(${variableName})
+    if(DEFINED ${variableName})
       string(REPLACE ";" "\;" value ${${variableName}})
       list(APPEND PASSTHROUGH_VARIABLES
         -D${variableName}=${value})