Add NOTES.UNIX, with a description on how to deal with runpaths

 For additional platform specific requirements, solutions to specific

 issues and other details, please read one of these:

  * NOTES.UNIX (any supported Unix like system)

  * NOTES.VMS (OpenVMS)

  * NOTES.WIN (any supported Windows)

  * NOTES.DJGPP (DOS platform with DJGPP)

 NOTES FOR UNIX LIKE PLATFORMS

 =============================

 For Unix/POSIX runtime systems on Windows, please see NOTES.WIN.

 Shared libraries and installation in non-standard locations

 -----------------------------------------------------------

 Binaries on Unix variants expect to find shared libraries in standard

 locations, such as /usr/lib, /usr/local/lib and some other locations

 configured in the system (for example /etc/ld.so.conf on some systems).

 If the libraries are installed in non-standard locations, binaries

 will not find them and therefore fail to run unless they get a bit of

 help from a defined RPATH or RUNPATH.  This can be applied by adding

 the appropriate linker flags to the configuration command, such as

 this (/usr/local/ssl was the default location for OpenSSL installation

 in versions before 1.1.0):

    $ ./config --prefix=/usr/local/ssl --openssldir=/usr/local/ssl \

        -Wl,-rpath,/usr/local/ssl/lib

 Because the actual library location may vary further (for example on

 multilib installations), there is a convenience variable in Makefile

 that holds the exact installation directory and that can be used like

 this:

    $ ./config --prefix=/usr/local/ssl --openssldir=/usr/local/ssl \

        -Wl,-rpath,'$(LIBRPATH)'