Set SWIGJOBS environment variable automatically

Use the number of available CPUs + 1 which seems to work best with the
number of actually available CPUs in the GitHub Actions environment.

.github/workflows/ci.yml

@@ -24,7 +24,6 @@ jobs:
 
     strategy:
       matrix:
-        #SWIGJOBS: ["-j2"]
         # other variables to optionally set
         # CC, GCC (used as suffix)
         # CPP11, CPP14, CPP17
@@ -47,7 +46,6 @@ jobs:
 
     env:
       SWIGLANG: ${{ matrix.SWIGLANG }}
-      SWIGJOBS: ${{ matrix.SWIGJOBS }}
       SWIG_FEATURES: ${{ matrix.SWIG_FEATURES }}
       CONFIGOPTS: ${{ matrix.CONFIGOPTS }}
       CC: ${{ matrix.CC }}
@@ -77,6 +75,22 @@ jobs:
           echo "${CONFIGOPTS[@]}"
           ./autogen.sh && mkdir -p build/build && cd build/build && ../../configure "${CONFIGOPTS[@]}"
 
+          case $(uname) in
+              Linux)
+                  cpu_count=$(nproc)
+                  ;;
+
+              Darwin)
+                  cpu_count=$(sysctl -n hw.ncpu)
+                  ;;
+
+              *)
+                  cpu_count=0
+                  ;;
+          esac
+          ((cpu_count++))
+          echo SWIGJOBS=-j$cpu_count >> $GITHUB_ENV
+
     - name: build
       shell: bash
       run: |