Use `${top_build_prefix}' for better compatibility with non-GNU make.

Use `${top_build_prefix}' for better compatibility with non-GNU make.
* libltdl/m4/ltdl.m4 (_LT_BUILD_PREFIX): New macro.
If the Autoconf version used is >= 2.62, then expand to
`${top_build_prefix}', otherwise to `${top_builddir}/'.
(LTDL_CONVENIENCE, LTDL_INSTALLABLE): Use it for defining
LIBLTDL.  Fixes a build failure with AIX make in a package
using convenience libltdl in nonrecursive mode.
* doc/libtool.texi (Distributing libltdl): Document requirements
to define `top_build_prefix' if Automake is not used.
Report by Bob Friesenhahn.