fix build with graphicsmagick

GM is still using the old page interface of subimage/subrange, IM has
deprecated that in favour of scene/number_scenes

we were accidentally just supporting the new IM system ... this change
adds support for GM as well, plus a configure test to pick one

see https://github.com/jcupitt/libvips/issues/423

ChangeLog

@@ -2,6 +2,7 @@
 - rename vips wrapper script, it was still vips-8.2, thanks Benjamin
 - export C++ operator overloads for MSVC linking [Lovell]
 - insist on giflib4
+- fix magickload @page with GraphicsMagick
 
 29/1/16 started 8.3
 - add vips_reduce*() ... a fast path for affine downsize

configure.ac

@@ -488,6 +488,19 @@ if test x"$with_magick" != "xno"; then
   LIBS=$save_LIBS
 fi
 
+if test x"$with_magick" != "xno"; then
+  # do we have number_scenes in image_info ... imagemagick uses this, gm
+  # still uses subrange
+  save_CFLAGS=$CFLAGS
+  CFLAGS="$CFLAGS $MAGICK_CFLAGS"
+  AC_CHECK_MEMBER([struct _ImageInfo.number_scenes],
+    AC_DEFINE(HAVE_NUMBER_SCENES,1,
+	      [define if your magick has ImageInfo.number_scenes.]),
+    [],
+    [#include <magick/api.h>])
+  CFLAGS=$save_CFLAGS
+fi
+
 # orc
 AC_ARG_WITH([orc], 
   AS_HELP_STRING([--without-orc], [build without orc (default: test)]))

libvips/foreign/magick2vips.c

@@ -49,6 +49,8 @@
  * 	  fd during file read, handy for large numbers of input images 
  * 14/2/16
  * 	- add @page option, 0 by default
+ * 18/4/16
+ * 	- fix @page with graphicsmagick
  */
 
 /*
@@ -217,16 +219,24 @@ read_new( const char *filename, VipsImage *im,
 #endif /*HAVE_SETIMAGEOPTION*/
 
 	if( !all_frames ) {
+#ifdef HAVE_NUMBER_SCENES 
+		 /* I can't find docs for these fields, but this seems to work.
+		  */
 		char page[256];
 
-		/* Just pick a specific page.
-		 *
-		 * I can't find docs for these fields, but this seems to work.
-		 */
 		read->image_info->scene = read->page;
 		read->image_info->number_scenes = 1;
+
+		/* Some IMs must have the string version set as well.
+		 */
 		vips_snprintf( page, 256, "%d", read->page );
 		read->image_info->scenes = strdup( page );
+#else /*!HAVE_NUMBER_SCENES*/
+		/* This works with GM 1.2.31 and probably others.
+		 */
+		read->image_info->subimage = read->page;
+		read->image_info->subrange = 1;
+#endif
 	}
 
 #ifdef DEBUG