Use eclipse compiler in Maven build

Use eclipse compiler in Maven build

Found instructions for configuring maven-compiler-plugin with ecj in
[1]. Verified that ecj run in this way raises build errors when executed
on commit d3ef5213.

Define profiles "ecj" for using Eclipse compiler and "javac" for using
javac including errorprone. By default ecj will be used.

use ecj:
$ mvn -Pecj clean install

use javac:
$ mvn -Pjavac clean install

TODO: find out how to run ecj with errorprone from Maven.

[1] https://stackoverflow.com/questions/33164976/using-eclipse-java-compiler-ecj-in-maven-builds

Change-Id: I716b603b57612b953e603387c82fd01eb1b5ca97
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>