README.md: Update according to ESP32 port

Surprisingly, esp-idf #includes files such as sys/types.h when pulling
other standard header files, even when not explicitly defined. While
this is not a serious issue, incredibly their sys/types.h implementation
uses "#define quad", therefore breaking user code such as anything
related to "struct quad".

README.md

@@ -112,7 +112,7 @@ building `enet`.
 A stripped version of the executable, as well as game assets, will be
 located in `build/cdimg`.
 
-#### Dependencies
+##### Dependencies
 
 A cross-compiled `i386-mingw32` version of all required dependencies
 is needed before building `jancity`.
@@ -126,6 +126,31 @@ of this writing, so it is provisionally provided on
 Read [the documentation](doc/BUILD-win9x.md) for further reference on
 how to build the dependencies from source.
 
+#### Espressif ESP32
+
+```sh
+ENETDIR=<enet-prefix> \
+    cmake -B build -DCMAKE_TOOLCHAIN_FILE=cmake/win9x-toolchain.cmake
+cmake --build build
+```
+
+Where:
+
+- `ENETDIR` is the path to the cross-compiled version for `enet`.
+
+##### Dependencies
+
+[`esp-idf`](https://github.com/espressif/esp-idf) is required to build this
+or any ESP32-related project. The `release/v5.1` branch is assumed here.
+As pointed out by
+[their documentation](https://docs.espressif.com/projects/esp-idf/en/v5.1.2/esp32/api-guides/build-system.html?highlight=idf_path#initialization),
+remember to assign the `IDF_PATH` environment variable to the directory
+where `esp-idf` is located.
+
+`esp-idf` requires to build `enet` using a custom CMake wrapper on top of it.
+For this project, this has been provided by the
+[`enet-esp32`](https://gitea.privatedns.org/xavi/enet-esp32) repository.
+
 ## License
 
 Unless stated otherwise, **jancity** follows the license described by the

src/gfx/esp32/inc/gfx/port.h

@@ -3,6 +3,8 @@
 
 #include <stdbool.h>
 
+#undef quad
+
 #ifdef __cplusplus
 extern "C"
 {

src/gfx/esp32/src/quad.c

@@ -4,6 +4,8 @@
 #include <stddef.h>
 #include <stdlib.h>
 
+#undef quad
+
 int quad_from_sprite(const struct sprite *const s, struct quad *const q)
 {
     return -1;

src/gfx/inc/gfx.h

@@ -1,9 +1,9 @@
 #ifndef GFX_H
 #define GFX_H
 
-#include <gfx/port.h>
 #include <stdbool.h>
 #include <stdio.h>
+#include <gfx/port.h>
 
 #ifdef __cplusplus
 extern "C"