Improve handling of system ICU on Linux

.github/workflows/Linux.yml

@@ -67,8 +67,8 @@ jobs:
           submodules: recursive
           fetch-depth: 0
 
-      - name: Install test dependencies
-        if: matrix.compiler != 'default'
+      - name: Install dependencies
+        if: matrix.compiler != ''
         run: |
           sudo apt-get install \
             libxkbcommon-x11-0 \
@@ -80,7 +80,8 @@ jobs:
             libxcb-render-util0 \
             libxcb-xinerama0 \
             libxcb-xfixes0 \
-            libegl1-mesa
+            libegl1-mesa \
+            libicu-dev
 
       - name: Install compiler
         id: install_compiler
@@ -128,8 +129,7 @@ jobs:
             -DCMAKE_BUILD_TYPE="RelWithDebInfo" \
             -DCMAKE_C_COMPILER_LAUNCHER="ccache" \
             -DCMAKE_CXX_COMPILER_LAUNCHER="ccache" \
-            -DCMAKE_INSTALL_PREFIX="../install" \
-            -DMLN_QT_WITH_INTERNAL_ICU=ON
+            -DCMAKE_INSTALL_PREFIX="../install"
           ninja
           ninja install
 

README.md

@@ -28,6 +28,14 @@ ninja
 ninja install
 ```
 
+### Linux
+
+Note that when using the system ICU library standalone Qt installation using
+installer is ignored. If you want to use that you need to make sure that your
+system ICU is not too new as it may prevent your app from running on older
+versions of Linux. Alternatively you can use internally bundled ICU with the
+`-DMLN_QT_WITH_INTERNAL_ICU=ON` CMake option.
+
 ### macOS
 
 Add the following arguments to the CMake call: