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>
---
Unchanged since v2.
ui-patch: use cgit_version not CGIT_VERSION

We already have a global cgit_version which is set from the #define'd
CGIT_VERSION in cgit.c.  Change ui-patch.c to use this so that we only
need to rebuild cgit.o when the version changes.

Signed-off-by: John Keeping <john@keeping.me.uk>
cgit.mk: don't rebuild everything if CGIT_VERSION changes

If CGIT_VERSION is in CGIT_CFLAGS then a change in version (for example
because you have committed your changes) causes all of the CGit objects
to be rebuilt.  Avoid this by using EXTRA_CPPFLAGS to add the version
for only those files that are affected and make them depend on VERSION.

Signed-off-by: John Keeping <john@keeping.me.uk>
---
Changes since v2:
- With the previous patch, ui-patch.c no longer references CGIT_VERSION
cgit.mk: Use SHELL_PATH_SQ to run gen-version.sh

On some platforms (notably Solaris) /bin/sh doesn't support enough of
POSIX for gen-version.sh to run.  Git's Makefile provides SHELL_PATH_SQ
to address this issue so we just have to use it.

Signed-off-by: John Keeping <john@keeping.me.uk>