Make the bundled bcrypt the default in Makefile.linux

Who am I kidding... at this time, the only viable option is OpenBSD's
own implementation. Perhaps if the future holds an acceptable and
widespread "libbcrypt", I revisit this.

Make it clear, that the bundled bcrypt is a "snapshot" of a version of
bcrypt_pbkdf() from the OBSD tree.

(cherry picked from commit 86f55378e1012d6ed7faea42e990044387a2a64e)
Release 2.4.0-rc3