wincred: align Makefile with other Makefiles in contrib

wincred: align Makefile with other Makefiles in contrib

* Replace $(LOADLIBES) because it is deprecated since long and it is
  used nowhere else in the git project.
* Use $(gitexecdir) instead of $(libexecdir) because config.mak defines
  $(libexecdir) as $(prefix)/libexec, not as $(prefix)/libexec/git-core.
* Similar to other Makefiles, let install target rule create
  $(gitexecdir) to make sure the directory exists before copying the
  executable and also let it respect $(DESTDIR).
* Shuffle the lines for the default settings to align them with the
  other Makefiles in contrib/credential.
* Define .PHONY for all special targets (all, install, clean).

Signed-off-by: Thomas Uhle <thomas.uhle@mailbox.tu-dresden.de>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>