contrib/credential: harmonize Makefiles

Update these Makefiles to be in line with other Makefiles from contrib
such as for contacts or subtree by making the following changes:

* Make the default settings after including config.mak.autogen and
  config.mak.
* Add the missing $(CPPFLAGS) to the compiler command as well as the
  missing $(CFLAGS) to the linker command.
* Use a pattern rule for compilation instead of a dedicated rule for
  each compile unit.
* Get rid of $(MAIN), $(SRCS) and $(OBJS) and simply use their values
  such as git-credential-libsecret and git-credential-libsecret.o.
* Strip @ from $(RM) to let the clean target rule be verbose.
* Define .PHONY for all special targets (all, clean).

Signed-off-by: Thomas Uhle <thomas.uhle@mailbox.tu-dresden.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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