Add --with-ext-lib-XXX support to SMB_EXT_LIB_FROM_PKGCONFIG
Where XXX is also the first argument to SMB_EXT_LIB_FROM_PKGCONFIG
e.g.
as
SMB_EXT_LIB_FROM_PKGCONFIG(LIBLDB, ....
exists in configure.ac it is now also possible to force selection
of the built-in LIBLDB, or the external LDB, or to prefer the
external LIBLDB if it is present by passing to ./configure
--with-ext-lib-LIBLDB
or
--without-ext-lib-LIBLDB
or
--with-ext-lib-LIBLDB=auto
The behavour defaults to "auto" but can be specified in a new
5th parameter to SMB_EXT_LIB_FROM_PKGCONFIG.