Debugger: Add compile time warning if cdb can't be found

Warn users who might have installed cdb in a none default path that
they have to define an environment variable during compile time.

Change-Id: Ic7e1cd02e596b9ad862214ac00ef319a76a6e7e1
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/libs.pro

@@ -38,5 +38,11 @@ win32:SUBDIRS += utils/process_ctrlc_stub.pro
 # Windows: Compile Qt Creator CDB extension if Debugging tools can be detected.
 win32: isEmpty(QTC_SKIP_CDBEXT) {
     include(qtcreatorcdbext/cdb_detect.pri)
-    exists($$CDB_PATH):SUBDIRS += qtcreatorcdbext
+    exists($$CDB_PATH) {
+        SUBDIRS += qtcreatorcdbext
+    } else {
+        message("Compiling Qt Creator without a CDB extension.")
+        message("If CDB is installed in a none default path define a CDB_PATH")
+        message("environment variable pointing to your CDB installation.")
+    }
 }