doc: split FreeBSD build instructions out of build-unix.md

docs: Linked to the 'Building on FreeBSD' section of the Unix guide where it lists BSD specific guides.

Created a FreeBSD build guide (doc/build-freebsd.md).

Added in warning about the version of 'gdb' installed by default.

Removed the FreeBSD build instructions now that they have their own guide (doc/build-freebsd.md).

Updated the sentence to refer to the BSD guides in the 'doc' directory for more specific BSD build instructions.

Minor grammatical fix.

doc/build-freebsd.md

@@ -0,0 +1,46 @@
+FreeBSD build guide
+======================
+(updated for FreeBSD 11.1)
+
+This guide describes how to build bitcoind and command-line utilities on FreeBSD.
+
+This guide does not contain instructions for building the GUI.
+
+## Preparation
+
+You will need the following dependencies, which can be installed as root via pkg:
+
+```
+pkg install autoconf automake boost-libs git gmake libevent libtool openssl pkgconf
+```
+
+For the wallet (optional):
+```
+./contrib/install_db4.sh `pwd`
+export BDB_PREFIX='$PWD/db4'
+```
+
+See [dependencies.md](dependencies.md) for a complete overview.
+
+Download the source code:
+```
+git clone https://github.com/bitcoin/bitcoin
+```
+
+## Building Bitcoin Core
+
+**Important**: Use `gmake` (the non-GNU `make` will exit with an error).
+
+```
+./autogen.sh
+
+./configure                  # to build with wallet OR
+./configure --disable-wallet # to build without wallet
+
+gmake
+```
+
+*Note on debugging*: The version of `gdb` installed by default is [ancient and considered harmful](https://wiki.freebsd.org/GdbRetirement).
+It is not suitable for debugging a multi-threaded C++ program, not even for getting backtraces. Please install the package `gdb` and
+use the versioned gdb command (e.g. `gdb7111`).
+