Add checks for newer JPEG API used by the escl backend. Fixes #243

The backend is excluded from the build if this API is not available by
SANE_CHECK_BACKENDS.

acinclude.m4

@@ -629,7 +629,14 @@ for be in ${BACKENDS}; do
     if test "x${sane_cv_use_libjpeg}" != "xyes"; then
       echo "*** $be backend currently requires JPEG library - $DISABLE_MSG"
       backend_supported="no"
+    else
+      if test "x${have_jpeg_crop_scanline}" != "xyes" \
+      || test "x${have_jpeg_skip_scanline}" != "xyes"; then
+        echo "*** $be backend requires a newer JPEG library - $DISABLE_MSG"
+        backend_supported="no"
+      fi
     fi
+
     ;;
 
     gphoto2)

configure.ac

@@ -695,14 +695,15 @@ else
   fi
 fi
 
-SANE_CHECK_BACKENDS
-
 if test "${sane_cv_use_libjpeg}" = "yes"; then
   SANEI_SANEI_JPEG_LO="../sanei/sanei_jpeg.lo"
+  AC_CHECK_FUNCS(jpeg_crop_scanline jpeg_skip_scanlines)
 fi
 AM_CONDITIONAL(HAVE_JPEG, test x$sane_cv_use_libjpeg = xyes)
 AC_SUBST(SANEI_SANEI_JPEG_LO)
 
+SANE_CHECK_BACKENDS
+
 AC_ARG_ENABLE(pnm-backend,
   AS_HELP_STRING([--enable-pnm-backend],
                  [enable the pnm backend for testing frontends (possible security risk, see PROBLEMS file)]),