Fix Windows/MinGW build: TLS, strndup, sys/wait.h

- CBM_TLS: use _Thread_local (C11 standard) instead of __declspec(thread)
  which only works on MSVC, not MinGW GCC
- Replace bare strndup() with cbm_strndup() in 6 files (strndup is POSIX,
  not available on Windows)
- Guard sys/wait.h with #ifndef _WIN32 in http_server.c