Update build instructions for cmake-based compilation.

Signed-off-by: DL6ER <dl6er@dl6er.de>

docs/ftldns/compile.md

@@ -1,51 +1,82 @@
 We pre-compile *FTL*DNS for you to save you the trouble of compiling anything yourself. However, sometimes you may want to make your own modifications. To test them, you have to compile *FTL*DNS from source. Luckily, you don't have to be a programmer to build *FTL*DNS from source and install it on your system; you only have to know the basics we provide in here. With just a few commands, you can build *FTL*DNS from source like a pro.
 
-#### Installing the Required Software
+## Installing the Required Software
 
 First, we'll install the basic software you'll need to compile from source, like the GCC compiler and other utilities.
 Install them by running the following command in a terminal:
 
-##### Debian / Ubuntu / Raspbian
+**Debian / Ubuntu / Raspbian**
 
 ```bash
-sudo apt install build-essential libgmp-dev m4
+sudo apt install build-essential libgmp-dev m4 cmake
 ```
 
-##### Fedora
+**Fedora**
 
 ```bash
-sudo dnf install gcc gmp-devel gmp-static m4
+sudo dnf install gcc gmp-devel gmp-static m4 cmake
 ```
 
----
+### Install `libnettle`
 
-You'll also need to compile `nettle` as *FTL*DNS uses `libnettle` for handling DNSSEC. Compile and install a recent version of `nettle` (we tested and recommend 3.6):
+*FTL*DNS uses a cryptographic library (`libnettle`) for handling DNSSEC signatures.
+Compile and install a recent version using:
 
 ```bash
 wget https://ftp.gnu.org/gnu/nettle/nettle-3.6.tar.gz
-tar -xvzf nettle-3.6.tar.gz
+tar -xzf nettle-3.6.tar.gz
 cd nettle-3.6
-./configure
-make
+./configure --libdir=/usr/local/lib
+make -j $(nproc)
 sudo make install
 ```
 
-#### Get the *FTL*DNS source
+Since Ubuntu 20.04, you need to specify the library directory explicitly. Otherwise, the library will be installed in custom locations where it would not be found by `cmake`.
+
+## Get the source
 
 Now, clone the *FTL*DNS repo (or your own fork) to get the source code of *FTL*DNS:
 
 ```bash
-git clone https://github.com/pi-hole/FTL.git
-cd FTL
+git clone https://github.com/pi-hole/FTL.git && cd FTL
 ```
 
-If you want to build another branch and not `master`, use checkout to get to this branch (e.g. `git checkout development`).
-
-*FTL*DNS can now be compiled and installed:
+If you want to build another branch and not `master`, use checkout to get to this branch, like
 
 ```bash
-make -j $(nproc)
-sudo make install
+git checkout development
+```
+
+## Compile the source
+
+*FTL*DNS can now be compiled using either the build script
+
+```bash
+./build.sh
+```
+
+or manually
+
+```bash
+mkdir -p cmake && cd cmake
+cmake ..
+cmake --build . -- -j $(nproc)
+```
+
+Note that both ways are exactly equivalent and that you do not need `root` priviledges here.
+
+## Install the new binary system-wide
+
+Install the new binary using either
+
+```bash
+./build.sh install
+```
+
+or
+
+```bash
+cd cmake && sudo make install
 ```
 
 Finally, restart *FTL*DNS to use the new binary: