doc: Add BUILDING.md: easy-to-read Linux/Ubuntu build & install guide

BUILDING.md

@@ -0,0 +1,88 @@
+# Building DeSmuME on Linux (Ubuntu)
+
+## 1. Install dependencies
+
+```bash
+sudo apt update
+sudo apt install -y \
+    build-essential git meson ninja-build \
+    libglib2.0-dev libsdl2-dev libpcap-dev zlib1g-dev \
+    libgtk-3-dev libx11-dev
+```
+
+**Optional packages** (auto-detected at configure time):
+
+| apt package           | Feature                          |
+|-----------------------|----------------------------------|
+| `libopenal-dev`       | OpenAL microphone input          |
+| `libasound2-dev`      | ALSA microphone input (fallback) |
+| `libsoundtouch-dev`   | High-quality audio resampling    |
+| `libagg-dev`          | HUD/OSD rendering                |
+| `libgl-dev`           | Desktop OpenGL rendering         |
+
+## 2. Get the source
+
+```bash
+git clone https://github.com/TASEmulators/desmume.git
+cd desmume
+```
+
+## 3. Build (Meson — GTK3 + CLI, recommended)
+
+```bash
+cd desmume/src/frontend/posix
+meson setup build --buildtype=release
+ninja -C build
+```
+
+Binaries after a successful build:
+- `build/gtk/desmume` — GTK3 graphical frontend
+- `build/cli/desmume-cli` — command-line frontend
+
+## 4. Install / Uninstall
+
+```bash
+sudo ninja -C build install    # installs to /usr/local by default
+sudo ninja -C build uninstall
+```
+
+To install to `/usr` instead of `/usr/local`:
+
+```bash
+meson setup build --buildtype=release --prefix=/usr
+sudo ninja -C build install
+```
+
+## 5. Optional build flags
+
+Pass `-Doption=true` to `meson setup`, e.g. `meson setup build -Dopengl=true`.
+
+| Flag                    | Default | Description                        |
+|-------------------------|---------|------------------------------------|
+| `-Dopengl=true`         | false   | Desktop OpenGL rendering           |
+| `-Dopengles=true`       | false   | OpenGL ES rendering                |
+| `-Dglx=true`            | false   | GLX context (requires OpenGL)      |
+| `-Degl=true`            | false   | EGL context                        |
+| `-Dopenal=true`         | false   | OpenAL microphone input            |
+| `-Dwifi=true`           | false   | Experimental Wi-Fi support         |
+| `-Dgdb-stub=true`       | false   | GDB stub for debugging             |
+| `-Dfrontend-gtk2=true`  | false   | GTK2 frontend (legacy)             |
+
+## 6. Legacy build (Autotools — GTK2)
+
+Use this only if you need the GTK2 frontend. Extra dependencies:
+
+```bash
+sudo apt install -y autoconf automake libgtk2.0-dev
+```
+
+```bash
+cd desmume/src/frontend/posix
+autoreconf -i   # or: ./autogen.sh
+./configure
+make -j$(nproc)
+```
+
+---
+
+More information: `desmume/README.LIN` · [Wiki](https://wiki.desmume.org/index.php?title=Installing_DeSmuME_from_source) · [Forum](https://forums.desmume.org)

README.md

@@ -4,3 +4,9 @@
 DeSmuME is a Nintendo DS emulator.
 
 http://desmume.org/download
+
+## Building from Source
+
+Pre-built binaries are available on the [download page](http://desmume.org/download).
+If you want to build DeSmuME yourself, see **[BUILDING.md](BUILDING.md)** for
+step-by-step instructions (Linux/Ubuntu, including required packages).