Jenkinsfile: Also use 4 parallel jobs for linux builds

Jenkinsfile

@@ -10,7 +10,7 @@ node('CLIENT') {
 		mkdir build
 		cd build
 		cmake -DUNIT_TESTING=1 -DBUILD_WITH_QT4=ON ..
-		make
+		make -j4
 		ctest --output-on-failure'''
 
     stage 'Qt4 - clang'
@@ -18,7 +18,7 @@ node('CLIENT') {
 		mkdir build
 		cd build
 		cmake -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DUNIT_TESTING=1 -DBUILD_WITH_QT4=ON ..
-		make
+		make -j4
 		ctest --output-on-failure'''
 
     stage 'Qt5'
@@ -26,7 +26,7 @@ node('CLIENT') {
 		mkdir build
 		cd build
 		cmake -DUNIT_TESTING=1 -DBUILD_WITH_QT4=OFF ..
-		make
+		make -j4
 		ctest --output-on-failure'''
 
     stage 'Qt5 - clang'
@@ -34,7 +34,7 @@ node('CLIENT') {
 		mkdir build
 		cd build
 		cmake -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DUNIT_TESTING=1 -DBUILD_WITH_QT4=OFF ..
-		make
+		make -j4
 		ctest --output-on-failure'''