Add X11BASE include directory
Index: GG/CMakeLists.txt
--- GG/CMakeLists.txt.orig
+++ GG/CMakeLists.txt
@@ -85,6 +85,7 @@ endif()
 include_directories(
     ${CMAKE_CURRENT_SOURCE_DIR}
     ${CMAKE_CURRENT_BINARY_DIR}
+    ${X11BASE}/include
 )
 
 configure_file(
