configure.ac: enable jpeglib support for pgtk
jpeglib を使えるようにした。
imagemagick がうまく機能してるので気付かなかった…
### Use -ljpeg if available, unless '--with-jpeg=no'.
HAVE_JPEG=no
LIBJPEG=
-if test "${HAVE_X11}" = "yes" || test "${HAVE_W32}" = "yes" \
+if test "${HAVE_X11}" = "yes" || test "$window_system" = "pgtk" || test "${HAVE_W32}" = "yes" \
|| test "${HAVE_NS}" = "yes"; then
if test "${with_jpeg}" != "no"; then
AC_CACHE_CHECK([for jpeglib 6b or later],