[ML] Preload mfront libraries on Linux.

Lars Bilkerequested to merge
bilke/ogs:mfront-runtime-fix into master

Somehow dlopen() does not successfully load dependencies of libOgsMFrontBehavior.so.

Workaround: Preload these dependencies with dlopen(). Then there are no LD_LIBRARY_PATH hacks required anymore. 🍻

Fixes #3073 (closed).

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