vips.h: problem with vips_init()

For version prior 7.41, vips_init() is a macro to VIPS_INIT().
VIPS_INIT() should be called, but as it is a macro, cgo could not
determine the return type, hence making the build fail.
So, we undef vips_init(), and define the vips_init() method.

vips.h

@@ -31,6 +31,25 @@ typedef struct {
 	double Background[3];
 } WatermarkOptions;
 
+/*
+ * This method is here to handle the weird initialization of the vips lib.
+ * libvips use a macro VIPS_INIT() that call vips__init() in version < 7.41,
+ * or calls vips_init() in version >= 7.41.
+ * Anyway, it's not possible to build bimg on Debian Jessie with libvips 7.40.x,
+ * as vips_init() is a macro to VIPS_INIT(), which is also a macro, hence, cgo
+ * is unable to determine the return type of vips_init(), making the build impossible.
+ * In order to correctly build bimg, for version < 7.41, we should undef vips_init and
+ * creates a vips_init() method that calls VIPS_INIT().
+ */
+#if (VIPS_MAJOR_VERSION == 7 && VIPS_MINOR_VERSION < 41)
+# undef vips_init
+int
+vips_init(const char *argv0)
+{
+	return VIPS_INIT(argv0);
+}
+#endif
+
 void
 vips_enable_cache_set_trace()
 {