Attempt to make travis build with more cores

.ci/script.sh

@@ -5,6 +5,8 @@ set -ex
 if [ "$TRAVIS_OS_NAME" = "linux" ]; then
     export CC=${C_COMPILER}
     export CXX=${CXX_COMPILER}
+    # make build use all available cores
+    export CMAKE_BUILD_PARALLEL_LEVEL=$(cat /proc/cpuinfo | awk '/^processor/{print $3}' | wc -l)
 
     sudo update-alternatives --install /usr/bin/gcc gcc "/usr/bin/${C_COMPILER}" 10
     sudo update-alternatives --install /usr/bin/g++ g++ "/usr/bin/${CXX_COMPILER}" 10