fftw-3: move configure.args-append down to be executed last

Previously, this was executed too early and appended the sse2 flags to
configure.args before the fftw-3-long subport had a chance to unset the
configure_args map.

Fixes: https://trac.macports.org/ticket/56146

math/fftw-3/Portfile

@@ -91,10 +91,6 @@ pre-configure {
             x86_64 "--enable-sse2"
         }
     }
-
-    if {![variant_isset universal]} {
-        configure.args-append $merger_configure_args(${configure.build_arch})
-    }
 }
 
 test.run     yes
@@ -214,12 +210,18 @@ if {[fortran_variant_isset]} {
     configure.args-delete --disable-fortran
 }
 
+# this is put at the bottom to run last (after the sub-port have done their own
+# logic)
 pre-configure {
     if {[mpi_variant_isset]} {
         # MPIRUN is needed for test phase
         configure.env           MPICC=${mpi.cc} MPIRUN=${mpi.exec}
         configure.args-append   --enable-mpi
     }
+
+    if {![variant_isset universal]} {
+        configure.args-append $merger_configure_args(${configure.build_arch})
+    }
 }
 
 if {${name} eq ${subport}} {