Merge #11945: Improve BSD compatibility of contrib/install_db4.sh

Merge #11945: Improve BSD compatibility of contrib/install_db4.sh

2712742 doc: Update FreeBSD build instructions to use bdb4 (Wladimir J. van der Laan)
d95c83d contrib: FreeBSD compatibility in install_db4.sh (Wladimir J. van der Laan)
c0298b0 contrib: Make X=Y arguments work in install_db4 (Wladimir J. van der Laan)
b798f9b contrib: New clang patch for install_db4 (Wladimir J. van der Laan)

Pull request description:

  This PR improves the BSD compatibility of the bdb4 installer script.

  See #11921, #11868.

  I've tested this on OpenBSD 6.2 (clang) and Ubuntu 16.04 (gcc).

  This needs testing on OSX at least, ~~and on gcc/Linux to make sure that applying the patch unconditionally doesn't negatively affect gcc~~.

  ~~NB: this is not yet sufficient to make `install_db4.sh` work on FreeBSD, as we need to use yet another `sha256` tool there. But it's a step in the right direction.~~

  ### contrib: New clang patch for install_db4

  Replace the clang patch with a new and improved version that also fixes the build issues with OpenBSD and FreeBSD's clang, and apply it unconditionally.

  Thanks to @fanquake for finding the patch.

  ### contrib: Make X=Y arguments work in install_db4

  Trailing X=Y arguments are supposed to be passed through unchanged to bdb's configure. This was not the case, at least with OpenBSD 6.2's shell.

  Fix this by not storing the arguments in a temporary variable but passing "$@" through directly.

  ### contrib: FreeBSD compatibility in install_db4.sh

  Unfortunately, FreeBSD uses yet another syntax for `sha256`.

  Support FreeBSD's syntax too. Using `uname` is a bit of a hack but it works and I found no way to distinguish the two.

Tree-SHA512: 12461a58dfeb4834701891762efc747c8187d834f41d98c8451edee1402a3958c4842bbc02c61bacbc7b0d90cc6b020a2ca158b65304d9760c9f0d2052ff36d4