• do not pre-fill HAVE_LINUX_TCP_H on Linux.
  linux/tcp.h is a Linux kernel userspace header. It's likely
  installed when using glibc and likely missing by default when using
  something else, e.g. MUSL (e.g. on Alpine).

  Therefore always detect it for Linux targets, and only pre-fill it for
  non-Linux ones.

• do not pre-fill HAVE_GLIBC_STRERROR_R on Linux.
  To fix it for non-glibc envs, e.g. MUSL (e.g. on Alpine).

Note, the pre-fill option is a disabled by default, internal option and
strongly not recommended outside of curl development.