Use separate $(MINSTALL) for installing libraries

Use separate $(MINSTALL) for installing libraries

The special feature of bin/minstall to copy symlinks is only ever needed
when installing libraries which may have .so symlinks. All the headers
and directories can use a normal install program.

These two modes are separated as $(INSTALL) and $(MINSTALL) to allow the
user (or autoconf) to override installing normal files as they please.
An autoconf check for the install program has been added and will be
used in preference to minstall when available.

Fixes bug 16053.