Motivation / Problem

The C++ std::getenv is guaranteed thread-safe by the C++11 specification,
whereas the POSIX/C getenv might not be thread-safe by the C11 specification.

Description

s/getenv/std::getenv/. There is no usage of setenv, putenv or unsetenv in our code, so that caveat in the specification should not apply to us.

Limitations

None.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker