Tor compilation fails when cross-compiling for macOS

Our latest nightly build is failing when (cross)-compiling tor for macOS:

  CC       src/lib/process/env.o
src/lib/process/env.c:40:11: warning: implicit declaration of function '_NSGetEnviron' is invalid in C99 [-Wimplicit-function-declaration]
  return *_NSGetEnviron();
          ^
src/lib/process/env.c:40:10: error: indirection requires pointer operand ('int' invalid)
  return *_NSGetEnviron();
         ^~~~~~~~~~~~~~~~
1 warning and 1 error generated.
Makefile:7498: recipe for target 'src/lib/process/env.o' failed