Allow to install a target without INCLUDE_DESTINATION (#33)

* Allow to install a target without `INCLUDE_DESTINATION`

Concrete use case:
A library like `gtest_main` that define a `main` symbols just need to be linked to.

* All `INCLUDE_DIR` to be empty too

CMakeLists.txt

@@ -165,13 +165,15 @@ function(packageProject)
     set(PROJECT_INCLUDE_HEADER_PATTERN "*")
   endif()
 
-  install(
-    DIRECTORY ${PROJECT_INCLUDE_DIR}/
-    DESTINATION ${PROJECT_INCLUDE_DESTINATION}
-    COMPONENT "${PROJECT_NAME}_Development"
-    FILES_MATCHING
-    PATTERN "${PROJECT_INCLUDE_HEADER_PATTERN}"
-  )
+  if(PROJECT_INCLUDE_DESTINATION AND PROJECT_INCLUDE_DIR)
+    install(
+      DIRECTORY ${PROJECT_INCLUDE_DIR}/
+      DESTINATION ${PROJECT_INCLUDE_DESTINATION}
+      COMPONENT "${PROJECT_NAME}_Development"
+      FILES_MATCHING
+      PATTERN "${PROJECT_INCLUDE_HEADER_PATTERN}"
+    )
+  endif()
 
   set(${PROJECT_NAME}_VERSION
       ${PROJECT_VERSION}