Makefile: define THIRD_PARTY_SOURCES

Makefile: define THIRD_PARTY_SOURCES

Some files in our codebase are borrowed from other projects, and
minimally updated to suit our own needs. We'd sometimes need to tell
our own sources and these third-party sources apart for management
purposes (e.g. we may want to be less strict about coding style and
other issues on third-party files).

Define the $(MAKE) variable THIRD_PARTY_SOURCES that can be used to
match names of third-party sources.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>