PROTON-2215: Windows build fails if CMAKE_MODULE_PATH is not empty (#141)

The previously used CMAKE_MODULE_PATH is actually a list of directories,
and therefore should not be used here.

Co-authored-by: Matthieu Longo <matthieu.longo@murex.com>
Co-authored-by: Jiri Danek <jdanek@redhat.com>
diff --git a/tools/cmake/Modules/WindowsC99CheckDef.cmake b/tools/cmake/Modules/WindowsC99CheckDef.cmake
index f1ecc88..6375c3b 100644
--- a/tools/cmake/Modules/WindowsC99CheckDef.cmake
+++ b/tools/cmake/Modules/WindowsC99CheckDef.cmake
@@ -30,5 +30,6 @@
     TARGET qpid-proton
 	PRE_LINK
     COMMAND ${PYTHON_EXECUTABLE}
-	    ${CMAKE_MODULE_PATH}/WindowsC99SymbolCheck.py $<TARGET_FILE_DIR:qpid-proton>
+            ${CMAKE_CURRENT_LIST_DIR}/WindowsC99SymbolCheck.py
+            $<TARGET_FILE_DIR:qpid-proton>
     COMMENT "Checking for dangerous use of C99-violating functions")