Support newer versions of javac

JamVM needs a java compiler when building for GNU Classpath, in
order to compile the VM support classes (classes.zip). Currently,
this breaks on JDK 9 or later, since the javac invocation uses
-bootclasspath, which (on JDK 9+) needs explicit -source/-target.

Fix by adding -source 1.6 -target 1.6 to javac options (same as
used by GNU Classpath itself).

While we are at this, also drop support for GCJ and Jikes. GCJ
was officially discontinued in 2016 (removed from GCC 7). Jikes
support was also removed from GNU Classpath long ago.

Signed-off-by: Guillermo Rodríguez <grodriguez@ingelabs.com>

Author: guillerodriguez

configure.ac

@@ -274,7 +274,7 @@ if test "$using_clang" = yes; then
     interp_cflags="$interp_cflags -mllvm -tail-dup-limit=0"
 fi
 
-AC_CHECK_PROGS(JAVAC, ecj jikes "gcj -C" javac)
+AC_CHECK_PROGS(JAVAC, ecj javac)
 
 dnl Checks for libraries.
 

src/classlib/gnuclasspath/lib/Makefile.am

@@ -50,7 +50,7 @@ inst_classes.zip: classes.zip
 
 classes.zip: $(JAVA_FILES)
 	-mkdir classes
-	$(JAVAC) -bootclasspath ${GLIBJ_ZIP} -d classes $(JAVA_FILES)
+	$(JAVAC) -source 1.6 -target 1.6 -bootclasspath ${GLIBJ_ZIP} -d classes $(JAVA_FILES)
 	cd classes && zip -r ../classes.zip .
 	rm -rf classes