allows `make check-header` to succeed when E-ACSL is compiled in internal mode

current dir might not be E-ACSL's main dir. For instance when the
plug-in is included in the main distribution 

- Disable on non-POSIX OSes
- Disable on anything that is not Linux or some BSD variant (excluding Darwin)
- Do not run extra checks if plug-in disabled

a plugin in the standard distribution must have `PLUGIN_DISTRIBUTED`
set to `yes` 

Integration within main distribution might work at some non-so-far-way
point in the future after all.