Derive JAVA_HOME from /usr/bin/javac

adaptors/java/src/Makefile

@@ -3,7 +3,7 @@ JARNAME=Moberg
 TARFILE=java_se.lth.control.realtime.Moberg-$(VERSION).tar.gz
 JNINAME=se_lth_control_realtime_moberg_Moberg
 
-JAVA_HOME=$(shell realpath $$(which javac) | sed 's|/bin/javac||')
+JAVA_HOME=$(shell realpath /usr/bin/javac | sed 's|/bin/javac||')
 JNI_INCLUDE=$(JAVA_HOME)/include/
 
 JNI_INCLUDE=$(JAVA_HOME)/include/