Always use sources instead of a hypothetical installed package.
diff '--color=auto' -uNr a/fortran/CMakeLists.txt b/fortran/CMakeLists.txt
--- a/fortran/CMakeLists.txt	2025-03-12 19:45:47.109635926 -0000
+++ b/fortran/CMakeLists.txt	2025-03-12 19:46:51.964594152 -0000
@@ -77,11 +77,8 @@
 
 # Get Spglib if it's run as stand-alone project
 if (NOT TARGET Spglib::symspg)
-    find_package(Spglib CONFIG)
-    if (NOT Spglib_FOUND)
         message(STATUS "Using bundled spglib sources")
         add_subdirectory(${PROJECT_SOURCE_DIR}/.. _deps/spglib-build)
-    endif ()
 endif ()
 
 #[=============================================================================[
diff '--color=auto' -uNr a/python/CMakeLists.txt b/python/CMakeLists.txt
--- a/python/CMakeLists.txt	2025-03-12 19:45:47.113635861 -0000
+++ b/python/CMakeLists.txt	2025-03-12 19:47:13.720244693 -0000
@@ -47,11 +47,8 @@
 
 # Get Spglib if it's run as stand-alone project
 if (NOT TARGET Spglib::symspg)
-    find_package(Spglib CONFIG)
-    if (NOT Spglib_FOUND)
         message(STATUS "Using bundled spglib sources")
         add_subdirectory(${PROJECT_SOURCE_DIR}/.. _deps/spglib-build)
-    endif ()
 endif ()
 
 #[=============================================================================[
diff '--color=auto' -uNr a/test/CMakeLists.txt b/test/CMakeLists.txt
--- a/test/CMakeLists.txt	2025-03-12 19:45:47.113635861 -0000
+++ b/test/CMakeLists.txt	2025-03-12 19:47:43.499766319 -0000
@@ -75,13 +75,11 @@
 
 # Get Spglib if it's run as stand-alone project
 if (NOT TARGET Spglib::symspg)
-    set(spglib_find_package_args)
     if (SPGLIB_WITH_Fortran)
         list(APPEND
                 COMPONENTS Fortran
         )
     endif ()
-    find_package(Spglib REQUIRED CONFIG ${spglib_find_package_args})
 endif ()
 
 set(BUILD_GMOCK OFF)