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>

contrib/credential/wincred/Makefile

@@ -4,20 +4,22 @@ all:: git-credential-wincred.exe
 -include ../../../config.mak.autogen
 -include ../../../config.mak
 
-CC ?= gcc
-RM ?= rm -f
-CFLAGS ?= -O2 -Wall
-
 prefix ?= /usr/local
-libexecdir ?= $(prefix)/libexec/git-core
+gitexecdir ?= $(prefix)/libexec/git-core
 
+CC ?= gcc
+CFLAGS ?= -O2 -Wall
 INSTALL ?= install
+RM ?= rm -f
 
-git-credential-wincred.exe : git-credential-wincred.c
-	$(LINK.c) $^ $(LOADLIBES) $(LDLIBS) -o $@
+git-credential-wincred.exe: git-credential-wincred.c
+	$(LINK.c) -o $@ $^ $(LDFLAGS) $(LDLIBS)
 
 install: git-credential-wincred.exe
-	$(INSTALL) -m 755 $^ $(libexecdir)
+	$(INSTALL) -d -m 755 $(DESTDIR)$(gitexecdir)
+	$(INSTALL) -m 755 $< $(DESTDIR)$(gitexecdir)
 
 clean:
 	$(RM) git-credential-wincred.exe
+
+.PHONY: all install clean