Fix annoying and otherwise unavoidable libtiff warning.

This warning appears in each buildbot build (which is wrong on its own as
libtiff configure shouldn't forcefully regenerate its output config.h every
time neither -- but currently it does) and is annoying as it distracts from
the other warnings, so suppress it even if usually we don't do it for the
warnings in 3rd party code.

src/tiff/libtiff/tif_open.c

@@ -89,6 +89,7 @@ TIFFClientOpen(
 	/* The following are configuration checks. They should be redundant, but should not
 	 * compile to any actual code in an optimised release build anyway. If any of them
 	 * fail, (makefile-based or other) configuration is not correct */
+#ifndef NDEBUG
 	assert(sizeof(uint8)==1);
 	assert(sizeof(int8)==1);
 	assert(sizeof(uint16)==2);
@@ -111,6 +112,7 @@ TIFFClientOpen(
 		assert(n.a16==1);
 		#endif
 	}
+#endif /* !NDEBUG */
 
 	m = _TIFFgetMode(mode, module);
 	if (m == -1)