Fix determining the number of logical CPUs under macOS

Use sysctrl instead of nproc which doesn't exist under Mac.

.github/workflows/ci.yml

@@ -119,7 +119,21 @@ jobs:
         run: |
           echo LD_LIBRARY_PATH=`pwd`/lib >> $GITHUB_ENV
 
-          wxPROC_COUNT=`nproc`
+          case `uname` in
+            Linux)
+                wxPROC_COUNT=`nproc`
+                ;;
+
+            Darwin)
+                wxPROC_COUNT=`sysctl -n hw.ncpu`
+                ;;
+
+            *)
+                echo "*** Unknown platform: `uname` ***"
+                wxPROC_COUNT=0
+                ;;
+          esac
+
           ((wxPROC_COUNT++))
           echo wxPROC_COUNT=$wxPROC_COUNT >> $GITHUB_ENV
           echo wxBUILD_ARGS=-j$wxPROC_COUNT >> $GITHUB_ENV