build/linux: Sync with beta manifest regarding x265

Ported from ca5688bf6d

Just to note, we are already not using Assembly code
in 10bit and 12bit libraries. So, no feature dropped.

---

Also, little fixes to some dev docs regarding Clang.

NEWS

@@ -135,14 +135,15 @@ Build:
   - Started a skeleton for unit testing framework for libgimp (only a few tests
     so far but we eventually target exhaustivity).
   - CI:
-    * Main CI jobs are now compiled with Clang, as well as Windows installer and
-      Nightly Flatpak builds. Some benchmarks run by Øyvind show we can gain a bit
+    * Main CI jobs are now compiled with Clang, as well as Windows installer,
+      MSIX and Flatpak. Some benchmarks run by Øyvind show we can gain a bit
       of performance in some situations in babl/GEGL/ctx codepaths, thanks to
       better auto-vectorization support.
     * x86 (32-bit) cross-build job for Windows removed.
     * "packaging" stage merged back with "build" stage.
     * "gimp-debian-x64" job now also generates an AppImage for every commit.
-    * LLD linker now used in Debian and flatpak jobs.
+    * LLD linker now used in all build jobs (except scheduled GCC and legacy
+      32-bit Windows jobs).
   - New gimp-data repository used as a git submodule:
     * Various data were moved to gimp-data: icons, cursors, patterns, brushes
       and application icon/logo.

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

@@ -578,11 +578,16 @@
                     "buildsystem": "cmake-ninja",
                     "subdir": "source",
                     "config-opts": [
+                        "-DCMAKE_CXX_COMPILER_LAUNCHER=ccache",
+                        "-DCMAKE_CXX_COMPILER=clang++",
+                        "-DCMAKE_CXX_FLAGS=-fuse-ld=lld",
                         "-DCMAKE_INSTALL_PREFIX=/app/lib/libheif",
                         "-DEXTRA_LIB='libx265-10.a;libx265-12.a'",
                         "-DEXTRA_LINK_FLAGS=-L/app/lib/libheif/lib",
                         "-DLINKED_10BIT=ON",
-                        "-DLINKED_12BIT=ON"
+                        "-DLINKED_12BIT=ON",
+                        "-DENABLE_CLI=OFF",
+                        "-DENABLE_ASSEMBLY=OFF"
                     ],
                     "cleanup": [
                         "/lib/libheif/bin",

devel-docs/README.md

@@ -327,7 +327,7 @@ running, such as:
 - Windows builds (cross compiled).
 
 Additionally, we test build with alternative tools or options (e.g. with
-`Clang` instead of `gcc` compiler) or jobs which may take much longer,
+`GCC` instead of `Clang` compiler) or jobs which may take much longer,
 such as package creation as scheduled pipelines (once every few days).
 
 The above listing is not necessarily exhaustive nor is it meant to be.