msvc: avoid collisions between "tags" and "TAGS"

Commit 2f769195 ("MinGW: avoid collisions between "tags" and "TAGS", 28-09-2010) enabled MinGW to use an ETAGS file in order to avoid filename collisions on (Windows) case insensitive filesystems. In addition, this prevents 'make' from issuing several warning messages. When using the Makefile to perform an MSVC build, which is usually executed using MinGW tools, we can also benefit from this capability. In order to reap the above benefits, we set the ETAGS_TARGET build variable to ETAGS in the MSVC config block. Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk> Tested-by: Johannes Sixt <j6t@kdbg.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>