Use infinitime_fonts target if fonts/CMakeLists.txt exists

Since https://github.com/InfiniTimeOrg/InfiniTime/pull/1097 the fonts
are generated. Support this new way of using fonts.

When InfiniTime isn't a subdirectory of InfiniSim we need to provide
the binary dir when using `add_subdirectory()`. Use `fonts`.

Update README with instructions to install `lv_font_conv`.

README.md

@@ -14,17 +14,18 @@ For a history on how this simulator started and the challenges on its way visit
 - CMake
 - SDL2 (provides the simulator window, handles mouse and keyboard input)
 - Compiler (g++ or clang++)
+- [lv_font_conv](https://github.com/lvgl/lv_font_conv#install-the-script) (for `font.c` generation since [InfiniTime#1097](https://github.com/InfiniTimeOrg/InfiniTime/pull/1097))
 
 On Ubuntu/Debian install the following packages:
 
 ```sh
-sudo apt install -y cmake libsdl2-dev g++
+sudo apt install -y cmake libsdl2-dev g++ npm
 ```
 
 On Arch Linux the following packages are needed:
 
 ```sh
-sudo pacman -S cmake sdl2 gcc
+sudo pacman -S cmake sdl2 gcc npm
 ```
 
 On Fedora the following packages are needed:
@@ -33,6 +34,12 @@ On Fedora the following packages are needed:
 sudo dnf install cmake SDL2-devel gcc
 ```
 
+Then install the `lv_font_conv` executable to the source directory (will be installed at `node_modules/.bin/lv_font_conv`)
+
+```sh
+npm install lv_font_conv@1.5.2
+```
+
 ## Get the Sources
 
 Clone this repository and tell `git` to recursively download the submodules as well