sparse: Fix errors due to missing target-specific variables

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 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.

Also, we change the 'check' target to re-direct the user to the
new sparse target.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>