CI: add build job for linux/GTK2

.github/workflows/build.yml

@@ -24,6 +24,24 @@ jobs:
       - name: ninja
         run: ninja -C desmume/src/frontend/posix/build
 
+  build_gtk2:
+    name: Build DeSmuME (Linux/GTK+2)
+    runs-on: ubuntu-22.04
+
+    steps:
+      - name: checkout
+        uses: actions/checkout@v2
+
+      - name: install dependencies
+        run: sudo apt update && sudo apt install autoconf libglu1-mesa-dev libsdl2-dev libpcap-dev libgtk2.0-dev
+
+      - name: buildit
+        run: |
+          cd desmume/src/frontend/posix/
+          autoreconf -i
+          ./configure --prefix=/usr --enable-gdb-stub --enable-wifi
+          make -j
+
   build_macos:
     name: Build DeSmuME (macOS)
     runs-on: macOS-11