build: Makefile fixes; let user give AR and RANLIB; local libtz.a

* Makefile: Let the user override AR, and build libtz.a before
installing it, as suggested by Michael Forney in
<https://github.com/eggert/tz/pull/3>.  Also, let the user
override RANLIB.  Modernize a bit by assuming POSIX.2-1992 or
later; this is safe nowadays and simplifies the libtz.a business.
And treat 'leapseconds' more like we used to, since it's
machine-independent a 'make clean' doesn't need to remove it.
(TZLIB): Remove.
(AR, RANLIB): New macros.
(TABDATA): Add leapseconds, so that 'make' builds it by default.
(DATA): Remove leapseconds, since $(TABDATA) now has it.
(all): Depend on libtz.a, not $(LIBOBJS).
(install): Install libtz.a using 'cp', like other files.
Also invoke $(RANLIB) on it, after installing.
Do not assume that TZDIR is not overridden.
(install, INSTALL, check_time_t_alternatives): Use 'mkdir -p', as
it's safe to assume nowadays (standardized in POSIX.2-1992).
(install, INSTALL): Prefer 'cp -f' to 'rm' followed by 'cp', as
it's safe to assume 'cp -f' nowadays (also standardized in POSIX.2-1992).
(libtz.a): Rename from $(DESTDIR)$(TZLIB), since we now build it here.
Use $(AR) rather than ar.
Use $(RANLIB) rather than trying to guess it.
(clean_misc): Do not remove 'leapseconds', as it's machine-independent.
(maintainer-clean): Remove 'leapseconds' here instead.
(clean, check_public): Standardize on 'rm -fr' rather than 'rm -f -r';
formerly the makefile was inconsistent.
(set-timestamps.out): Don't ignore failurs of the actual touch command.