Remove bash-ism from configure script

--- configure.orig	2025-10-07 03:17:25 UTC
+++ configure
@@ -22303,7 +22303,7 @@ rm -f core conftest.err conftest.$ac_objext conftest.b
 
   if test "X$flag_ok" = Xyes ; then
 
-                  CYTHON_CFLAGS+=" -Wno-cast-function-type"
+                  CYTHON_CFLAGS="$CYTHON_CFLAGS -Wno-cast-function-type"