Mac 11 support, beside mac 10 (#22)

.github/workflows/build.yml

@@ -71,11 +71,16 @@ jobs:
 # TODO: Add OSX build.	   Problem: stdfwd.hh has to be conditionally disabled for _APPLE_ (huge but simple)
 # TODO: Add Windows build. Problem: EnjoLibBoost fails to compile without wstring (short but harder)
 
-
   build-macos:
-    runs-on: macOS-latest
+    runs-on: ${{ matrix.os }}
     env:
       CCACHE_TEMPDIR: /tmp/.ccache-temp
+    strategy:
+      fail-fast: false
+      matrix:
+        #os: [macOS-latest, macOS-11]    # prev
+        #os: [macOS-latest, macOS-10.15] # future 
+        os: [macOS-10.15, macOS-11]      # transitionary
     steps:
     - uses: actions/checkout@v1
       with:
@@ -85,8 +90,8 @@ jobs:
       uses: actions/cache@v2
       with:
         path: /Users/runner/Library/Caches/ccache
-        key:          ccache03-${{ runner.os }}-build-${{ github.sha }}
-        restore-keys: ccache03-${{ runner.os }}-build-
+        key:          ccache03-${{ runner.os }}-${{ matrix.os }}-build-${{ github.sha }}
+        restore-keys: ccache03-${{ runner.os }}-${{ matrix.os }}-build-
     # Less volatile cache
     - name: data cache
       uses: actions/cache@v2

README.md

@@ -37,10 +37,11 @@ In case these instructions become outdated, please refer to the steps of the [CI
 
 ## Requirements
 Supported systems:
-- Debian-stable
-- Ubuntu 20.04 & 21.04
-- Mac OSX
+- Debian stable & buster
+- Ubuntu 21.04 & 20.04(!)
+- Mac OSX 11(!) & 10.15(!)
 - ~~Windows~~ (somewhat later)
+- (!) = under CI
 
 ## Preparation
 Please run the below scripts. They are meant to be non-interactive and will require root permissions (via `sudo`).