Permit to specify directory where to load plugins from without using LD_LIBRARY_PATH/PATH

[traversaro]

See discussion robotology/robotology-superbuild#95 for more info.

[drdanz]

Related: ami-iit/sharedlibpp#7

[traversaro]

For qpsolvers-eigen I implemented yet another improvement, that I think is quite convenient and permits us to avoid setting any env variables, at least in most cases.

The main library is called <prefix>qpsolvers-eigen<extension> (where <prefix> and <extension> are os-specific values), while plugins are called in <prefix>qpsolvers-eigen-osqp<extension>, ``qpsolvers-eigen-proxqpetc etc. Differently from rtf that installs the main library in/lib` (or `/bin` for Windows) and the plugins in `/lib/rtf`, for qpsolvers all libraries are installed in `/lib`, as anyhow they are not ambiguous thanks to the `qpsolvers-eigen-` prefix in their name.

Then, in the main library I extend the search path based on the location of the main <prefix>qpsolvers-eigen<extension> library (see https://github.com/ami-iit/qpsolvers-eigen/blob/dfd9b3e326e27f8f73646070d120c53e42c4fbda/core/QpSolversEigen/Solver.cpp#L102-L110 and https://github.com/ami-iit/qpsolvers-eigen/blob/dfd9b3e326e27f8f73646070d120c53e42c4fbda/core/QpSolversEigen/Solver.cpp#L21-L57), so that as long as the library and the plugins are installed in the same install prefix (as it happens with conda packages, robotology-superbuild and colcon workspace with --merge-install option) the plugins can be found without the need of setting any env variable.

In case plugins are installed in a separate install prefix with respect to the main <prefix>qpsolvers-eigen<extension> library, then the install directory can be added to the SHLIBPP_PLUGIN_PATH env variable (or a custom env variable as done by blockfactory with BLOCKFACTORY_PLUGIN_PATH, see https://github.com/robotology/blockfactory/blob/9921b59725b31cc5a655b9bdb316c634df6085d8/sources/Core/src/FactorySingleton.cpp#L30 .