Avoid warnings on unused CMAKE_BUILD_TYPE on Windows (#487) (#488)

Signed-off-by: Jose Luis Rivero <[email protected]>
(cherry picked from commit e6097c5)

Co-authored-by: Jose Luis Rivero <[email protected]>

Author: mergify[bot]

CMakeLists.txt

@@ -259,3 +259,9 @@ gz_create_docs(
   API_MAINPAGE_MD "${CMAKE_BINARY_DIR}/api.md"
   TUTORIALS_MAINPAGE_MD "${CMAKE_BINARY_DIR}/tutorials.md"
   )
+
+# Workaround to avoid warnings when using CMAKE_BUILD_TYPE in colcon
+# to build other projects and gz-cmake together on Windows
+if(DEFINED CMAKE_BUILD_TYPE)
+  set(_dummy_var "${CMAKE_BUILD_TYPE}")
+endif()