make the name of the library directory a config option

make the name of the library directory a config option

Introduce new makefile variable lib to hold the name of the lib
directory ("lib" by default).  Also introduce a switch for configure
to specify this name with --with-lib=ARG.  This is useful for systems
that use a different name than "lib" (like "lib64" on some 64 bit
Linux architectures).

Signed-off-by: Robert Schiele <rschiele@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>