Add BSD build notes and dependencies

Also makes a couple minor compatibility fixes:

 - OpenBSD's thread name in Loguru

 - Update headers for the BSDs

docs/build-freebsd.md

@@ -0,0 +1,53 @@
+# Building on FreeBSD
+
+FreeBSD builds can be created with Clang using the Meson build system.
+
+Before getting started, you should know that FreeBSD isn't fully supported or
+used by the current maintainers. It's been lightly tested on a big-endian
+PowerPC system.
+
+## Pre-requisites
+
+### Setup your `bin` path
+
+In order to run Python PIP-installed programs, we link `~/.local/bin` to
+`~/bin`, which is in FreeBSD's default user `PATH`:
+
+``` shell
+ln -s .local/bin ~/bin
+```
+
+## Build instructions
+
+Clone and enter the repository:
+
+``` shell
+git clone https://github.com/dosbox-staging/dosbox-staging.git
+cd dosbox-staging
+```
+
+Install dependencies:
+
+``` shell
+$ su (enter root password)
+# pkg install `cat packages/freebsd.txt`
+# exit (back to your user account)
+$ pip install meson
+```
+
+Build:
+
+``` shell
+meson setup build
+ninja -C build
+```
+
+To learn more about Meson's build options, see [BUILD.md](/BUILD.md).
+
+## Run
+
+Your binary is `build/dosbox`, which you can link into your `~/bin/` path:
+
+```shell
+ln -s $PWD/build/dosbox ~/bin/dosbox-staging
+```

docs/build-openbsd.md

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+# Building on OpenBSD
+
+OpenBSD builds can be created with Clang using the Meson build system.
+
+Before getting started, you should know that OpenBSD isn't fully supported or
+used by the current maintainers. It's been lightly tested on a big-endian
+PowerPC system.
+
+## Pre-requisites
+
+### Expand the systems' memory limits
+
+The LLVM link process will map more virtual memory than OpenBSD normally grants
+to user processes, so it's recommended to open up these limits by editing
+`/etc/login.conf` with your root account:
+
+Change the two default `datasize-` fields to `infinity`, as follows:
+
+```
+default:\
+  ...
+  :datasize-max=infinity:\
+  :datasize-cur=infinity:\
+  ...
+```
+
+This change allowed the link process to successfully run on an Apple Powerbook
+G4 with only 2 GB of physical RAM.
+
+### Setup your `bin` path
+
+In order to run Python PIP-installed programs, we link `~/.local/bin` to
+`~/bin`, which is in OpenBSD's default user `PATH`:
+
+``` shell
+ln -s .local/bin ~/bin
+```
+
+### Consider a `wxallowed` mount-point on slow hardware
+
+If your platform is old or slow (such as 32-bit PowerPC, MIPS, or
+32-bit ARM SBC), then you might benefit by disabling per-page W^X
+permission flagging.
+
+If so:
+
+ 1) Read the about the per-page W^X permission build option in the
+    `meson_options.txt`, and consider comparing builds.
+
+ 2) If you do make a build without per-page W^X (`-Dper_page_w_or_x=disabled`)
+    your binary needs to reside within a filesystem mounted with the
+    `wxallowed` flag. This is OpenBSD-specific, where as the other BSDs
+    don't have this requirement.
+
+    The good news is that OpenBSD mounts `/usr/local` with the `wxallowed`
+    flag by default, so the easiest option is to simply place your
+    build tree under there, for example: `/usr/local/src/dosbox-staging`.
+
+    Alternatively, you can edit `/etc/fstab` and add the `wxallowed`
+    mount flag the filesystem where your build tree resides.
+
+If you're on a fast 64-bit platform or _are_ using per-page W^X, then
+your binary will run anywhere and you can skip this.
+
+
+## Build instructions
+
+Clone and enter the repository:
+
+``` shell
+git clone https://github.com/dosbox-staging/dosbox-staging.git
+cd dosbox-staging
+```
+
+Install dependencies:
+
+``` shell
+pkg_add $(cat packages/openbsd.txt)
+pip install meson
+```
+
+Build:
+
+``` shell
+meson setup build
+ninja -C build
+```
+
+To learn more about Meson's build options, see [BUILD.md](/BUILD.md).
+
+## Run
+
+Your binary is `build/dosbox`, which you can link into your `~/bin/` path:
+
+```shell
+ln -s $PWD/build/dosbox ~/bin/dosbox-staging
+```