support building plugins without ltdl

[Redfoxymoon]

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Description

support building imagemagick with shared modules without using ltdl, allows building imagemagick with slibtool and for platforms that ltdl does not support

missing configure changes

[dlemstra]

Not sure if we should support this? Do you have examples of platforms where this now doesn't work? And if we do this it might be better to create new defines that either use dlclose or lt_dlclose and do the same for the other methods like dlerror?

[Redfoxymoon]

Midipix, a POSIX layer for microsoft windows lacks support for ltdl, we already have imagemagick running there with dynamic plugins. update: slibtool supports (s)ltdl

a set of macros would remove the need for most of the ifdef preprocessor changes, true; that's another option

[dlemstra]

This would also require changes to the configure.ac file to detect either libltdl or dlfcn and sets flags accordingly. And I think libltdl should be preferred when that could be found.

[Redfoxymoon]

Defaulting to libltdl would be fine - alternatively a set of --enable/--disable- configure options?

[orbea]

@dlemstra Would this be acceptable with a set of --enable/--disable configure flags as suggested by @Redfoxymoon?

This would be helpful on Gentoo when using slibtool instead of GNU libtool which doesn't install the libtool archive (.la) files by default.