Change to out-of source build, Update build instructions (#3271)

* update build instructions

Usage of "modern" CMake syntax and using specific out of source build.
Further add example on how to build in parallel, build a specific target
and how to change the compiler.

This closes #3236.

performance/run_perf

@@ -16,7 +16,7 @@ fi
 # Allow override.
 if [[ -z $TASK ]]
 then
-  TASK=../src/task
+  TASK=../build/src/task
 fi
 
 # Run benchmarks.