Enable LTO in release builds

Fixes #693

.github/actions/install_deps.sh

@@ -12,3 +12,5 @@ case `echo $1 | cut -d '-' -f 1` in
 esac
 
 bison --version
+make --version
+cmake --version

.github/workflows/testing.yml

@@ -34,7 +34,7 @@ jobs:
         if: matrix.buildsys == 'make'
       - name: Build & install using CMake
         run: |
-          cmake -S . -B build -DCMAKE_VERBOSE_MAKEFILE=ON ${{ matrix.cmakevars }}
+          cmake -S . -B build -DCMAKE_VERBOSE_MAKEFILE=ON -DCMAKE_BUILD_TYPE=Release ${{ matrix.cmakevars }}
           cmake --build build
           cp build/src/rgb{asm,link,fix,gfx} .
           sudo cmake --install build