ProjectExplorer: Convert empty macro to "KEY="

-DKEY is the same as #define KEY 1
-DKEY= is the same as #define KEY

Change-Id: I958a89d7b024999f1a623d4d309513ce342f6f8a
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/projectexplorer/projectmacro.cpp

@@ -109,6 +109,8 @@ QByteArray Macro::toKeyValue(const QByteArray &prefix) const
         keyValue = prefix;
 
     if (value.isEmpty())
+        keyValue += key + '=';
+    else if (value == "1")
         keyValue += key;
     else
         keyValue += key + '=' + value;