gnuplot: opportunistically uses gtk2 when building with wxwidgets enabled

[dbevans (David B. Evans)]

I ran into this issue when testing p5-graphics-gnuplotif.

When building using the +wxwidgets variant (default), gnuplot will use gtk2 if available and enable "gdk/gtk tweaks" to the configuration. This could be fixed by adding a dependency on gtk2 but that may not be what you want here. The alternative seems to be patching or otherwise overriding configure.in to disable this behavior -- there's no configuration option to do this for you.

Here's the relevant section of configure.in:

if test "${enable_wxwidgets_ok}" = yes ; then
  WX_CXXFLAGS="`$WX_CONFIG --cxxflags | sed 's/-fno-exceptions//'` $CAIROPANGO_CFLAGS"
  WX_LIBS="`$WX_CONFIG --libs` $CAIROPANGO_LIBS"

  dnl Check for fork(), used for the 'persist' effect
  AC_FUNC_FORK

  dnl Check for gtk (raise/lower tweaks)
  PKG_CHECK_MODULES(GTK, [gtk+-2.0], have_gtk=yes, have_gtk=no)
  if  test "${have_gtk}" = yes ; then
    AC_DEFINE(HAVE_GTK, 1, [Define to use gtk/gdk tweaks])
    WX_CXXFLAGS="$WX_CXXFLAGS $GTK_CFLAGS"
    WX_LIBS="$WX_LIBS $GTK_LIBS"
  fi