Align shlibpp with the version vendored in blockfactory and add support for loading plugins by looking in SHLIBPP_PLUGIN_PATH env variable

[traversaro]

Align shlibpp with the changes done in blockfactory in the following PRs and commits:

• Support BLOCKFACTORY_PLUGIN_PATH environment variable robotology/blockfactory#32, in particular robotology/blockfactory@9f527ea, robotology/blockfactory@f76da67, robotology/blockfactory@c8adc06 and robotology/blockfactory@04b9bed
• robotology/blockfactory@1a43005#diff-cda776c6821be73142659b5a7fde043453a27e48646d2974263556a87ca82751
• Include What You Use: Always include cstdint when necessary robotology/blockfactory#72

The main change introduced is the feature of looking for plugin names by automatically adding the os-specific prefix and extensions, and looking for them in the directories specified in SHLIBPP_PLUGIN_PATH, or let downstream project specify the project-specific environment variable, as done in blockfactory with the BLOCKFACTORY_PLUGIN_PATH env variable.