macOS: optionally make a generically-named copy of the zip (#6518)

Adds `-DOSX_NIGHTLY`. With -DOSX_NIGHTLY=ON will produce copies of the zip with generic names: `RawTherapee_macOS_arm64_latest.zip` for the purpose of hyperlinking. This is activated for GitHub Actions automated builds.

* mac: specify catalina on github action
* mac: use clang instead of /path/to/clang
* mac: Exec Architectures --> Architecture Priority
* mac: add an option to generate a universal app
* mac: generate a universal app
* mac: use "Universal" for arch in naming
* mac: remove some braces
* mac: fix gtk3 environment paths
* mac: fix universal url logic
* mac: merge about txts for universal build
* Mac: fix osx version minimums in universal 1
* Mac: update info plist for osx version minima
* Mac: update universal versioning
* Mac: add processor tunings for sandy-ivy bridge
* mac: fix minimum versioning statements
* Mac: fix a paste error
* mac: remove any x/X libs from bundle
* mac: don't get rid of libxcb for cairo
* mac: fix a typo
* mac: revert library removals for cairo

.github/workflows/main.yml

@@ -14,7 +14,7 @@ on:
       - created
 jobs:
   build:
-    runs-on: macos-latest
+    runs-on: macos-10.15
     steps:
       - uses: actions/checkout@v2
       - name: Install dependencies
@@ -52,8 +52,8 @@ jobs:
             -DPROC_LABEL="generic processor" \
             -DWITH_LTO="OFF" \
             -DLENSFUNDBDIR="/Applications/RawTherapee.app/Contents/Resources/share/lensfun" \
-            -DCMAKE_C_COMPILER=/usr/local/opt/llvm/bin/clang \
-            -DCMAKE_CXX_COMPILER=/usr/local/opt/llvm/bin/clang++ \
+            -DCMAKE_C_COMPILER=clang \
+            -DCMAKE_CXX_COMPILER=clang++ \
             -DCMAKE_C_FLAGS="-arch x86_64 -Wno-pass-failed -Wno-deprecated-register -Wno-unused-command-line-argument" \
             -DCMAKE_CXX_FLAGS="-arch x86_64 -Wno-pass-failed -Wno-deprecated-register -Wno-unused-command-line-argument" \
             -DOpenMP_C_FLAGS="${C_FLAGS}" \