In order to be cached, configure variables need to contain the
string '_cv_', and they should begin with a package-specific
prefix in order to avoid interfering with third-party macros.
Rename ld_dashr, ld_wl_rpath, ld_rpath to git_cv_ld_dashr etc.
Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>