Makefile: re-use Git's Makefile where possible

Git does quite a lot of platform-specific detection in its Makefile,
which can result in it defining preprocessor variables that are used in
its header files.  If CGit does not define the same variables it can
result in different sizes of some structures in different places in the
same application.

For example, on Solaris Git uses it's "compat" regex library which has a
different sized regex_t structure than that available in the platform
regex.h.  This has a knock-on effect on the size of "struct rev_info"
and leads to hard to diagnose runtime issues.

In order to avoid all of this, introduce a "cgit.mk" file that includes
Git's Makefile and make all of the existing logic apply to CGit's
objects as well.  This is slightly complicated because Git's Makefile
must run in Git's directory, so all references to CGit files need to be
prefixed with "../".

In addition, OBJECTS is a simply expanded variable in Git's Makefile so
we cannot just add our objects to it.  Instead we must copy the two
applicable rules into "cgit.mk".  This has the advantage that we can
split CGit-specific CFLAGS from Git's CFLAGS and hence avoid rebuilding
all of Git whenever a CGit-specific value changes.

Signed-off-by: John Keeping <john@keeping.me.uk>
Acked-by: Jamie Couture <jamie.couture@gmail.com>

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