--- a/m4/ax_lib_fftw3.m4	2026-09-15 09:01:23.181964635 +0200
+++ b/m4/ax_lib_fftw3.m4	2026-09-15 09:04:28.721957025 +0200
@@ -80,7 +80,8 @@
       fftw3_lib_flag=""
       fftw3_LIBS=""
     else
-      fftw3_libname="${!fftw3_libvar:2}"
+      eval "fftw3_lib_flag=\$${fftw3_libvar}"
+      fftw3_libname="${fftw3_lib_flag#??}"
       fftw3_lib_flag="-l${fftw3_libname}"
       fftw3_LIBS="${fftw3_lib_flag} -lm"
     fi