Fix Makefile for Apple Silicon M1 and add CI build

.github/workflows/ci.yml

@@ -35,3 +35,20 @@ jobs:
       run: make -C GUI/xephem
     - name: Test
       run: make -C tests
+
+  compile-apple-silicon:
+    runs-on: macos-14
+    steps:
+    - uses: actions/checkout@v2
+    - name: Install OpenMotif
+      run: brew install openmotif && brew info openmotif
+    - name: Install groff
+      run: brew install groff && brew info groff
+    - name: Install OpenSSL
+      run: brew install openssl && brew info openssl
+    - name: Install XQuartz
+      run: brew install xquartz --cask && brew info xquartz
+    - name: Compile
+      run: make -C GUI/xephem
+    - name: Test
+      run: make -C tests

GUI/xephem/Makefile

@@ -29,11 +29,16 @@ MOTIFL = /usr/lib
 endif
 
 # For Linux and Apple OS X
+# /opt/homebrew is for modern Apple Silicon; the other paths, for older macOS.
 
 UNAME_S = $(shell uname -s)
 ifeq ($(UNAME_S),Darwin)
-  PLATI = -I/usr/local/opt/openssl@3/include -I/opt/X11/include
-  PLATL = -L/usr/local/opt/openssl@3/lib -L/opt/X11/lib
+  PLATI = -I/opt/homebrew/include \
+          -I/usr/local/opt/openssl@3/include \
+          -I/opt/X11/include
+  PLATL = -L/opt/homebrew/lib \
+          -L/usr/local/opt/openssl@3/lib \
+          -L/opt/X11/lib
 endif
 
 CC = gcc