https://bugs.gentoo.org/913518
https://github.com/FFTW/fftw3/issues/130
https://github.com/FFTW/fftw3/pull/338

From f69fef7aa546d4477a2a3fd7f13fa8b2f6c54af7 Mon Sep 17 00:00:00 2001
From: Tobias Mayer <tobim@fastmail.fm>
Date: Tue, 24 Oct 2023 13:46:33 +0200
Subject: [PATCH] Fix the cmake config file when configured with autotools

When the autotools based build is used the FFTW3ConfigDeps.cmake file is
not generated, so including it fails with an error. By making the
include optional, the config file can still be used in a limited
fashion.
--- a/FFTW3Config.cmake.in
+++ b/FFTW3Config.cmake.in
@@ -10,7 +10,7 @@ set (FFTW3@PREC_SUFFIX@_LIBRARIES fftw3@PREC_SUFFIX@)
 set (FFTW3@PREC_SUFFIX@_LIBRARY_DIRS @CMAKE_INSTALL_FULL_LIBDIR@)
 set (FFTW3@PREC_SUFFIX@_INCLUDE_DIRS @CMAKE_INSTALL_FULL_INCLUDEDIR@)
 
-include ("${CMAKE_CURRENT_LIST_DIR}/FFTW3LibraryDepends.cmake")
+include ("${CMAKE_CURRENT_LIST_DIR}/FFTW3LibraryDepends.cmake" OPTIONAL)
 
 if (CMAKE_VERSION VERSION_LESS 2.8.3)
   set (CMAKE_CURRENT_LIST_DIR)