Only add subdirectories if necessary

If the build is using system libraries (for bzip2, lua, ode, or
xdgbasedir), there's no need to add the corresponding subdirectories.
This reduces build time and avoids potential mix-ups.

Signed-off-by: Stephen Kitt <steve@sk2.org>

src/CMakeLists.txt

@@ -65,13 +65,21 @@ find_package(ZLIB REQUIRED)
 
 # sources
 
-add_subdirectory("${PROJECT_SOURCE_DIR}/vendor/bzip2" bzip2)
-add_subdirectory("${PROJECT_SOURCE_DIR}/vendor/libccd" libccd)
+if(NOT USE_SYSTEM_Bzip2)
+  add_subdirectory("${PROJECT_SOURCE_DIR}/vendor/bzip2" bzip2)
+endif()
 add_subdirectory("${PROJECT_SOURCE_DIR}/vendor/chipmunk" chipmunk)
-add_subdirectory("${PROJECT_SOURCE_DIR}/vendor/lua" lua)
+if(NOT USE_SYSTEM_Lua)
+  add_subdirectory("${PROJECT_SOURCE_DIR}/vendor/lua" lua)
+endif()
 add_subdirectory("${PROJECT_SOURCE_DIR}/vendor/md5sum" md5sum)
-add_subdirectory("${PROJECT_SOURCE_DIR}/vendor/ode" ode)
-add_subdirectory("${PROJECT_SOURCE_DIR}/vendor/xdgbasedir" xdgbasedir)
+if(NOT USE_SYSTEM_ODE)
+  add_subdirectory("${PROJECT_SOURCE_DIR}/vendor/libccd" libccd)
+  add_subdirectory("${PROJECT_SOURCE_DIR}/vendor/ode" ode)
+endif()
+if(NOT USE_SYSTEM_XDG)
+  add_subdirectory("${PROJECT_SOURCE_DIR}/vendor/xdgbasedir" xdgbasedir)
+endif()
 add_subdirectory("${PROJECT_SOURCE_DIR}/vendor/glad" glad)
 
 set(common_src