Allow libtiff_support_custom_tags to be missing

[radarhere]

Resolves #7019

Image.core.libtiff_support_custom_tags may be missing, if libtiff is unavailable.

Pillow/src/_imaging.c

Lines 4256 to 4272 in 2d5f451

	#ifdef HAVE_LIBTIFF
	{
	extern const char *ImagingTiffVersion(void);
	PyDict_SetItemString(
	d, "libtiff_version", PyUnicode_FromString(ImagingTiffVersion()));
	// Test for libtiff 4.0 or later, excluding libtiff 3.9.6 and 3.9.7
	PyObject *support_custom_tags;
	#if TIFFLIB_VERSION >= 20111221 && TIFFLIB_VERSION != 20120218 && \
	TIFFLIB_VERSION != 20120922
	support_custom_tags = Py_True;
	#else
	support_custom_tags = Py_False;
	#endif
	PyDict_SetItemString(d, "libtiff_support_custom_tags", support_custom_tags);
	}
	#endif