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