OS checks with configuration time DLOPEN_NEEDS_LIBDL

[Richard PALO]

Since GUIBASE was replaced with the equivalent OS checks, there seems
to be a bit of abandon in keeping multiple OSes up to snuff.

in https://gerrit.libreoffice.org/#/c/18591/  I took our base in pkgsrc and
given the biggest difference in unix systems is the need for -ldl or not for dlopen,

I added a configuration check for DLOPEN_NEEDS_LIBDL and updated a number of the existing
OS checks to check simultaneously for, among others, %BSD and SOLARIS in addition to LINUX.

It would be nice to get some review on this approach.

-- 
Richard PALO