build/linux: Build flatpak with GCC to avoid LLVM/freedesktop branch hell

Despite building GIMP with Clang helped a lot to make our deps builds robust,
it is very hard to maintain, not because of LLVM but because of freedesktop.
We needed to ensure that the branch of LLVM matched the one of freedesktop on
which GNOME SDK is based. That stopped to work resulting on bad inconsistency:
1) flathub never had a problem; 2) GNOME runners sometimes worked, others not;
3) locally it stopped to work completely, so the .json right now is unportable.

Some annoying bugs were:
- "error: Requested extension *llvm* not installed"
- "Similar refs found for *llvm*"

So, let's end this not using any extension anymore while keeping only the nice
improvements in the custom builds.

build/linux/flatpak/1_build-deps-flatpak.sh

@@ -30,8 +30,6 @@ if [ -z "$GITLAB_CI" ]; then
   flatpak remote-add --if-not-exists --user --from gnome-nightly https://nightly.gnome.org/gnome-nightly.flatpakrepo
   flatpak install --user gnome-nightly org.gnome.Platform/$(uname -m)/master org.gnome.Sdk/$(uname -m)/master -y
 fi
-flatpak remote-add --if-not-exists --user --from flathub https://dl.flathub.org/repo/flathub.flatpakrepo
-flatpak install --user flathub org.freedesktop.Sdk.Extension.llvm18 -y
 
 
 # Prepare env (only GIMP_PREFIX is needed for flatpak)

build/linux/flatpak/org.gimp.GIMP-nightly.json

@@ -4,22 +4,15 @@
     "runtime": "org.gnome.Platform",
     "runtime-version": "master",
     "sdk": "org.gnome.Sdk",
-    "sdk-extensions": [
-        "org.freedesktop.Sdk.Extension.llvm18"
-    ],
     "command": "gimp",
     "build-options": {
-        "append-path": "/usr/lib/sdk/llvm18/bin",
-        "prepend-ld-library-path": "/usr/lib/sdk/llvm18/lib",
         "env": {
-            "CC": "ccache clang",
-            "CXX": "ccache clang++",
-            "CC_LD": "lld",
-            "CXX_LD": "lld",
+            "CC": "ccache cc",
+            "CXX": "ccache c++",
             "CLICOLOR_FORCE": "1"
         },
-        "cflags": "-fcolor-diagnostics",
-        "cxxflags": "-fcolor-diagnostics"
+        "cflags": "-fdiagnostics-color=always",
+        "cxxflags": "-fdiagnostics-color=always"
     },
     "separate-locales": false,
     "rename-desktop-file": "gimp.desktop",
@@ -569,8 +562,7 @@
                     "buildsystem": "cmake-ninja",
                     "config-opts": [
                         "-DCMAKE_CXX_COMPILER_LAUNCHER=ccache",
-                        "-DCMAKE_CXX_COMPILER=clang++",
-                        "-DCMAKE_CXX_FLAGS=-fuse-ld=lld",
+                        "-DCMAKE_CXX_COMPILER=c++",
                         "-DCMAKE_INSTALL_PREFIX=/app/lib/libheif",
                         "-DEXTRA_LIB='libx265-10.a;libx265-12.a'",
                         "-DEXTRA_LINK_FLAGS=-L/app/lib/libheif/lib",