Introduce git_etc_gitconfig() that encapsulates access of ETC_GITCONFIG.

Introduce git_etc_gitconfig() that encapsulates access of ETC_GITCONFIG.

In a subsequent patch the path to the system-wide config file will be
computed. This is a preparation for that change. It turns all accesses
of ETC_GITCONFIG into function calls. There is no change in behavior.

As a consequence, config.c is the only file that needs the definition of
ETC_GITCONFIG. Hence, -DETC_GITCONFIG is removed from the CFLAGS and a
special build rule for config.c is introduced. As a side-effect, changing
the defintion of ETC_GITCONFIG (e.g. in config.mak) does not trigger a
complete rebuild anymore.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>