ci: Only use Linux clang for app image build. (#7244)

* ci: Only use Linux clang for app image build.

* build: Re-add -Wno-attributes for GCC 11.

.ci/linux.sh

@@ -1,13 +1,15 @@
-#!/bin/sh -ex
+#!/bin/bash -ex
+
+if [ "$TARGET" = "appimage" ]; then
+    export COMPILER_FLAGS=(-DCMAKE_CXX_COMPILER=clang++ -DCMAKE_C_COMPILER=clang -DCMAKE_LINKER=/etc/bin/ld.lld)
+fi
 
 mkdir build && cd build
 cmake .. -G Ninja \
     -DCMAKE_BUILD_TYPE=Release \
     -DCMAKE_C_COMPILER_LAUNCHER=ccache \
     -DCMAKE_CXX_COMPILER_LAUNCHER=ccache \
-    -DCMAKE_CXX_COMPILER=clang++ \
-    -DCMAKE_C_COMPILER=clang \
-    -DCMAKE_LINKER=/etc/bin/ld.lld \
+    "${COMPILER_FLAGS[@]}" \
     -DENABLE_QT_TRANSLATION=ON \
     -DCITRA_ENABLE_COMPATIBILITY_REPORTING=ON \
     -DENABLE_COMPATIBILITY_LIST_DOWNLOAD=ON \

src/CMakeLists.txt

@@ -124,6 +124,12 @@ else()
         add_compile_options("-stdlib=libc++")
     endif()
 
+    if (CMAKE_CXX_COMPILER_ID STREQUAL GNU)
+        # GCC may warn when it ignores attributes like maybe_unused,
+        # which is a problem for older versions (e.g. GCC 11).
+        add_compile_options("-Wno-attributes")
+    endif()
+
     if (MINGW)
         add_definitions(-DMINGW_HAS_SECURE_API)
         if (COMPILE_WITH_DWARF)