autotools: cleanup lastlog2 stuff

autotools: cleanup lastlog2 stuff

* use pkg-config (PKG_CHECK_MODULES) rather than check for sqlite3.h
  only
* define $SQLITE3_LIBS

* use UL_ macros in configure.ac for liblastlog2 and pam_lastlog2

* add --disable-pam-lastlog2

* check for PAM to define BUILD_PAM_LASTLOG2

* install lastlog2.h to $(includedir)/liblastlog2/lastlog2.h

* fix liblastlog2_la_LDFLAG

* add Requires.private to lastlog2.pc.in

* use $(SQLITE3_LIBS) rather than hardcoded -lsqlite3

Fixes: https://github.com/util-linux/util-linux/issues/2734
Signed-off-by: Karel Zak <kzak@redhat.com>