Make sure rJava installation is as user-friendly as possible

com.oracle.truffle.r.pkgs/rJava/src/Makevars

 JAVA_SRC=$(wildcard java/*.java)
 JFLAGS=-source 1.6 -target 1.6
-JAVAC=javac
+JAVAC=${JAVA_HOME}/bin/javac
 
 all: $(SHLIB)
 $(SHLIB): java
@@ -8,7 +8,7 @@ $(SHLIB): java
 .PHONY: all java
 
 java: $(JAVA_SRC)
-	$(JAVAC) $(JFLAGS) $(JAVA_SRC)
+	$(JAVAC) $(JFLAGS) $(JAVA_SRC) || (echo "ERROR: compilation failed, do you have properly configured JAVA_HOME environment variable?"; exit 1)
 	rm -rfv ../inst/java
 	mkdir -p ../inst/java
 	mv java/*.class ../inst/java