Re: [PATCH 7/9] sparse: Fix errors due to missing target-specific variables

> In particular, sparse issues the following errors:
>
>     attr.c:472:43: error: undefined identifier 'ETC_GITATTRIBUTES'
>     config.c:821:43: error: undefined identifier 'ETC_GITCONFIG'
>     exec_cmd.c:14:37: error: undefined identifier 'PREFIX'
>     exec_cmd.c:83:28: error: undefined identifier 'GIT_EXEC_PATH'
>     builtin/help.c:328:46: error: undefined identifier 'GIT_MAN_PATH'
>     builtin/help.c:374:40: error: undefined identifier 'GIT_INFO_PATH'
>     builtin/help.c:382:45: error: undefined identifier 'GIT_HTML_PATH'
>     git.c:96:42: error: undefined identifier 'GIT_HTML_PATH'
>     git.c:241:35: error: invalid initializer
>     http.c:293:43: error: undefined identifier 'GIT_HTTP_USER_AGENT'
>
> which is caused by not passing the target-specific additions to
> the EXTRA_CPPFLAGS variable to cgcc.
>
> In order to fix the problem, we define a new sparse target which
> depends on an a set of non-existent "sparse object" files (*.sp)
> which correspond to the set of C source files. In addition to the
> new target, we also provide a new pattern rule for "creating" the
> sparse object files from the source files by running cgcc.  This
> allows us to add '*.sp' to the rules setting the target-specific
> EXTRA_CPPFLAGS variable, which is then included in the new pattern
> rule to run cgcc.