fix build with imagemagick 6.7.8.9

Ships with centos7, so an important version to support.

See https://github.com/libvips/libvips/issues/1479

ChangeLog

@@ -1,4 +1,5 @@
 17/9/19 started 8.8.4
+- improve compatibility with older imagemagick versions
 
 31/8/19 started 8.8.3
 - revert sharpen restoring the input colourspace

configure.ac

@@ -698,6 +698,16 @@ if test x"$magick6" = x"yes"; then
     ]
   )
 
+  AC_COMPILE_IFELSE(
+    [AC_LANG_PROGRAM(
+      [#include <magick/api.h>],
+      [ColorspaceType colorspace = HCLpColorspace]
+    )],
+    [AC_DEFINE(HAVE_HCLPCOLORSPACE,1,
+         [define if your Magick has HCLpColorspace.])
+    ]
+  )
+
   # GetImageMagick() takes two args under GM, three under IM
   AC_COMPILE_IFELSE(
     [AC_LANG_PROGRAM(

libvips/foreign/magick.c

@@ -515,21 +515,24 @@ static MagickColorspaceTypeNames magick_colorspace_names[] = {
 #ifdef HAVE_CMYCOLORSPACE
 	{ CMYColorspace, "CMYColorspace" },
 	{ HCLColorspace, "HCLColorspace" },
-	{ HCLpColorspace, "HCLpColorspace" },
 	{ HSBColorspace, "HSBColorspace" },
+	{ LabColorspace, "LabColorspace" },
+	{ LogColorspace, "LogColorspace" },
+	{ LuvColorspace, "LuvColorspace" },
+#endif /*HAVE_CMYCOLORSPACE*/
+
+#ifdef HAVE_HCLPCOLORSPACE
+	{ HCLpColorspace, "HCLpColorspace" },
 	{ HSIColorspace, "HSIColorspace" },
 	{ HSVColorspace, "HSVColorspace" },
-	{ LabColorspace, "LabColorspace" },
 	{ LCHColorspace, "LCHColorspace" },
 	{ LCHabColorspace, "LCHabColorspace" },
 	{ LCHuvColorspace, "LCHuvColorspace" },
-	{ LogColorspace, "LogColorspace" },
 	{ LMSColorspace, "LMSColorspace" },
-	{ LuvColorspace, "LuvColorspace" },
 	{ scRGBColorspace, "scRGBColorspace" },
 	{ xyYColorspace, "xyYColorspace" },
 	{ YDbDrColorspace, "YDbDrColorspace" },
-#endif /*HAVE_CMYCOLORSPACE*/
+#endif /*HAVE_HCLPCOLORSPACE*/
 
 	/* im7 has this, I think
 	 *