build: Update cmake files to use variables as destinations for make install

Change-Id: Ic1d6ebdb73e796cbb1207963973e410c9928c04f
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 34bbf9a..4cd4d30 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -289,7 +289,7 @@
 endif()
 
 if(UNIX)
-    install(DIRECTORY "${PROJECT_SOURCE_DIR}/include/vulkan" DESTINATION include)
+    install(DIRECTORY "${PROJECT_SOURCE_DIR}/include/vulkan" DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
 endif()
 
 # loader: Generic VULKAN ICD loader