project file allow installation in another dir than "/usr"

E.g. this way I can build and install in "~/.local", or "/usr/local".

.github/workflows/appimage.yml

@@ -0,0 +1,98 @@
+name: "ci"
+
+on:
+  push:
+    tags: "**"
+  workflow_dispatch:
+
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  appimage:
+    runs-on: ubuntu-18.04
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v2
+      with:
+        submodules: true
+
+    - name: Install dependencies
+      run: |
+        sudo add-apt-repository -y ppa:beineri/opt-qt-5.15.2-bionic
+        sudo apt-get update -qq
+        sudo apt-get install -y \
+        build-essential make wget libgl1-mesa-dev \
+        qt515declarative qt515graphicaleffects \
+        qt515quickcontrols qt515quickcontrols2
+
+    - name: Download QT appimage builder
+      run: |
+        wget -c -O linuxdeployqt.AppImage \
+        https://github.com/probonopd/linuxdeployqt/releases/download/continuous/linuxdeployqt-continuous-x86_64.AppImage
+        chmod a+x linuxdeployqt.AppImage
+
+    - name: Build project
+      run: |
+        source /opt/qt*/bin/qt*-env.sh && \
+        qmake -v && \
+        qmake CONFIG+=release PREFIX=/usr && \
+        make -j$(nproc)
+
+    - name: Install to appdir
+      run: |
+        source /opt/qt*/bin/qt*-env.sh && \
+        make INSTALL_ROOT=appdir -j$(nproc) install
+
+    - name: Extract version number
+      run: |
+        # Extract version for linuxdeployqt to name the file. Use the tag as
+        # release name but remove prefix.
+        echo "VERSION=$(echo '${{ github.ref }}' | sed 's;.*/;;')" >> $GITHUB_ENV
+
+    - name: Build appimage directory
+      run: |
+        mkdir -p \
+        appdir/usr/bin \
+        appdir/usr/lib \
+        appdir/usr/share/applications \
+        appdir/usr/share/metainfo \
+        appdir/usr/share/icons/hicolor/128x128/apps
+
+        cp cool-retro-term appdir/usr/bin/
+        cp cool-retro-term.desktop appdir/usr/share/applications/
+        cp packaging/appdata/cool-retro-term.appdata.xml appdir/usr/share/metainfo/
+        cp app/icons/128x128/cool-retro-term.png appdir/usr/share/icons/hicolor/128x128/apps/
+        cp -r ./app/qml appdir/usr/
+        # Workaround for https://github.com/probonopd/linuxdeployqt/issues/78
+        cp -r ./qmltermwidget/QMLTermWidget appdir/usr/qml/
+
+        find appdir | sort
+
+    - name: Build appimage
+      run: |
+        source /opt/qt*/bin/qt*-env.sh && \
+        ./linuxdeployqt.AppImage appdir/usr/share/applications/cool-retro-term.desktop \
+        -verbose=1 -appimage \
+        -qmldir=./app/qml/ \
+        -qmldir=./qmltermwidget/
+      env:
+        # Unset environment variables
+        QTDIR:
+        QT_PLUGIN_PATH:
+        LD_LIBRARY_PATH:
+
+    - name: Upload release
+      uses: softprops/action-gh-release@v1
+      with:
+        body: appimage release
+        files: ./**/Cool_Retro_Term-*-x86_64.AppImage
+
+    - name: Clean up
+      if: always()
+      run: |
+        find appdir -executable -type f -exec ldd {} \; | grep " => /usr" | cut -d " " -f 2-3 | sort | uniq
+        make clean
+        rm -rf appdir

app/app.pro

@@ -16,23 +16,29 @@ macx:ICON = icons/crt.icns
 RESOURCES += qml/resources.qrc
 
 #########################################
-##              INTALLS
+##              INSTALLS
 #########################################
 
-target.path += /usr/bin/
+PREFIX = $$(PREFIX) # Pass the make install PREFIX via environment variable. E.g. "PREFIX=/path/to/my/dir qmake".
+isEmpty(PREFIX) {
+    message(No prefix given. Using /usr.)
+    PREFIX=/usr
+}
+
+target.path = $$PREFIX/bin
 
 INSTALLS += target
 
 # Install icons
 unix {
     icon32.files = icons/32x32/cool-retro-term.png
-    icon32.path = /usr/share/icons/hicolor/32x32/apps
+    icon32.path = $$PREFIX/share/icons/hicolor/32x32/apps
     icon64.files = icons/64x64/cool-retro-term.png
-    icon64.path = /usr/share/icons/hicolor/64x64/apps
+    icon64.path = $$PREFIX/share/icons/hicolor/64x64/apps
     icon128.files = icons/128x128/cool-retro-term.png
-    icon128.path = /usr/share/icons/hicolor/128x128/apps
+    icon128.path = $$PREFIX/share/icons/hicolor/128x128/apps
     icon256.files = icons/256x256/cool-retro-term.png
-    icon256.path = /usr/share/icons/hicolor/256x256/apps
+    icon256.path = $$PREFIX/share/icons/hicolor/256x256/apps
 
     INSTALLS += icon32 icon64 icon128 icon256
 }

cool-retro-term.pro

@@ -5,7 +5,17 @@ CONFIG += ordered
 SUBDIRS += qmltermwidget
 SUBDIRS += app
 
+#########################################
+##              INSTALLS
+#########################################
+
+PREFIX = $$(PREFIX) # Pass the make install PREFIX via environment variable. E.g. "PREFIX=/path/to/my/dir qmake".
+isEmpty(PREFIX) {
+    message(No prefix given. Using /usr.)
+    PREFIX=/usr
+}
+
 desktop.files += cool-retro-term.desktop
-desktop.path += /usr/share/applications
+desktop.path += $$PREFIX/share/applications
 
 INSTALLS += desktop