Make PACKAGE_PREFIX_DIR workaround more robust

packaging/NetlibF2CConfig.cmake.in

@@ -25,7 +25,11 @@ SOFTWARE.
 
 @PACKAGE_INIT@
 
+# TODO: Remove, when CMAKE_MINIMUM_REQUIRED_VERSION >= 3.29.1
+# See https://gitlab.kitware.com/cmake/cmake/-/issues/25827
+if (DEFINED PACKAGE_PREFIX_DIR)
     set(NetlibF2C_PACKAGE_PREFIX_DIR "${PACKAGE_PREFIX_DIR}")
+endif ()
 
 cmake_minimum_required(VERSION @CMAKE_MINIMUM_REQUIRED_VERSION@)
 
@@ -51,7 +55,11 @@ set(
 
 find_dependency(LoadStaticSharedTargets REQUIRED CONFIG)
 
+# TODO: Remove, when CMAKE_MINIMUM_REQUIRED_VERSION >= 3.29.1
+# See https://gitlab.kitware.com/cmake/cmake/-/issues/25827
+if (DEFINED NetlibF2C_PACKAGE_PREFIX_DIR)
     set(PACKAGE_PREFIX_DIR "${NetlibF2C_PACKAGE_PREFIX_DIR}")
+endif ()
 
 include(LoadStaticSharedTargets)