Please add env/envFile support for LanguageServer

Type: General
Describe the bug

• OS and Version: Ubuntu 19.10
• VS Code Version: 1.41.0
• C/C++ Extension Version: 0.26.2

Related issues: #4691, #4178, #3078

Description
It is common that compiler requires extra environment variables to run correctly (PATH, LD_LIBRARY_PATH, even less-known CPATH in my case) which are provided by some kind of "activation script".

Without proper setting of such variables detecting compiler's default include paths and defines doesn't work (or doesn't work as expected).

Proposal
env / envFile settings are getting more and more wide-spread and it would be awesome if cpptools respects them when detecting compiler and its settings (even via compile_commands.json).
Having such option per-configuration and per-project significantly improves experience when developer needs to switch between different compilers.

Unfortunately I couldn't find place in the repo where compiler is invoked...

Inizia individuando il percorso di rilevamento del compilatore di LanguageServer e la sua gestione di compile_commands.json; l’issue non indica file o test del repository. Confronta il comportamento richiesto di env/envFile per configurazione e per progetto con le impostazioni esistenti, e definisci come done il rilevamento corretto dei percorsi di inclusione predefiniti e dei defines del compilatore negli ambienti attivati.