Use $BACKEND instead of BDB2.  Fallback to LDBM if $BACKEND is not BDB2.

Integration into ./configure ( --enable-bdb2 ).
Adaptation of the test-suite ( 'make' -> ldbm; 'make bdb2-local' -> bdb2 ).
Minor fixes in slapd/libldbm.

might have in their ldap.conf(5) file.

  make depend, make tests, and make install all work when build directory
  is not the $srcdir.
Also modified library handling such that -lpthread more likely to be last.
WARNING: new orderring requires use of LDFLAGS to set global loader options
  such as -L/usr/local/lib.  If you put this in LIBS, some libraries
  may not be found a link time.
Likely broke Kerberos/LDAPD support.  Don't have those in my testbed.