how to create a patch for configure?

[macports at parvis.nl]

Working on my missing dejavusansmono in rrdtool pango cairo fontconfig problem:

pango calls 'FcWeightFromOpenTypeDouble' an that function exists only in fontconfig >= 2.92

That implies changes to pango or fontconfig and I can't oversee the consequences.

But I verified pango source and it doesn't really use that function but still the older 'FcWeightFromOpenType'.
That means that if I bypass the configure check everything should work fine.

pango configure:18606
---------------------
  if $have_fontconfig ; then
    pango_save_libs=$LIBS
    LIBS="$LIBS $FONTCONFIG_LIBS"
    for ac_func in FcWeightFromOpenTypeDouble
  do :
    ac_fn_c_check_func "$LINENO" "FcWeightFromOpenTypeDouble" "ac_cv_func_FcWeightFromOpenTypeDouble"
  if test "x$ac_cv_func_FcWeightFromOpenTypeDouble" = xyes; then :
    cat >>confdefs.h <<_ACEOF
  #define HAVE_FCWEIGHTFROMOPENTYPEDOUBLE 1
  _ACEOF

this check fails so HAVE_FCWEIGHTFROMOPENTYPEDOUBLE is still undefined. The result is that pango doesn't use fontconfig.

pango configure.ac:221
----------------------
  if $have_fontconfig ; then
    pango_save_libs=$LIBS
    LIBS="$LIBS $FONTCONFIG_LIBS"
    AC_CHECK_FUNCS(FcWeightFromOpenTypeDouble)
    LIBS=$pango_save_libs
  fi

here I want to disable AC_CHECK_FUNCS() and also add #define HAVE_FCWEIGHTFROMOPENTYPEDOUBLE 1 to confdefs.h

I made a patch resulting in

pango configure.ac:221
----------------------
    if $have_fontconfig ; then
      pango_save_libs=$LIBS
      LIBS="$LIBS $FONTCONFIG_LIBS"
->    # AC_CHECK_FUNCS(FcWeightFromOpenTypeDouble)
->    cat >>confdefs.h <<_ACEOF
->  #define HAVE_FCWEIGHTFROMOPENTYPEDOUBLE 1
->  _ACEOF
      LIBS=$pango_save_libs
    fi

but Autoconf didn't run so configure was not updated so nothing changes.

Q1: if the patch result correct?
Q2: How should I do this in Portfile ?