[WiP] Use lcms2 fast-float plugin

configure.ac

@@ -791,6 +791,18 @@ if test x"$with_lcms" != x"no"; then
   )
 fi
 
+# fast-float plugin (GPL3) added in 2.10.
+if test x"$with_lcms" != x"no"; then
+  save_CFLAGS="$CFLAGS"
+  CFLAGS="$CFLAGS $LCMS_CFLAGS"
+  AC_CHECK_DECL(LCMS2_FAST_FLOAT_VERSION,[
+     AC_DEFINE(HAVE_LCMS2_FAST_FLOAT_PLUGIN,1,
+	       [define if the fast-float plugin of lcms2 is available.])
+   ],[],
+   [#include <lcms2_fast_float.h>])
+  CFLAGS="$save_CFLAGS"
+fi
+
 # we need a conditional for this to only compile in fallback profiles if lcms
 # is detected
 AM_CONDITIONAL(ENABLE_LCMS, [test x"$with_lcms" != x"no"])

libvips/colour/icc_transform.c

@@ -87,6 +87,10 @@
  */
 #include <lcms2.h>
 
+#ifdef HAVE_LCMS2_FAST_FLOAT_PLUGIN
+#include <lcms2_fast_float.h>
+#endif
+
 #include <vips/vips.h>
 
 #include "pcolour.h"
@@ -207,14 +211,32 @@ is_pcs( cmsHPROFILE profile )
 		cmsGetColorSpace( profile ) == cmsSigXYZData ); 
 }
 
+#ifdef HAVE_LCMS2_FAST_FLOAT_PLUGIN
+static void *
+vips_icc_fast_float_init_cb( void *a )
+{
+	cmsPlugin( cmsFastFloatExtensions() );
+
+	return( NULL );
+}
+#endif
+
 static int
 vips_icc_build( VipsObject *object )
 {
+#ifdef HAVE_LCMS2_FAST_FLOAT_PLUGIN
+	static GOnce once = G_ONCE_INIT;
+#endif
+
 	VipsObjectClass *class = VIPS_OBJECT_GET_CLASS( object ); 
 	VipsColour *colour = (VipsColour *) object;
 	VipsColourCode *code = (VipsColourCode *) object;
 	VipsIcc *icc = (VipsIcc *) object;
 
+#ifdef HAVE_LCMS2_FAST_FLOAT_PLUGIN
+	VIPS_ONCE( &once, vips_icc_fast_float_init_cb, NULL ); 
+#endif
+
 	if( icc->depth != 8 &&
 		icc->depth != 16 ) {
 		vips_error( class->nickname,