JDK17 gives warnings about synchronization and deprecation (for removal).
While here, push the source/target to 8 which is what JDK21 will need.

diff --git a/usr/src/lib/libslp/javalib/Makefile.conf b/usr/src/lib/libslp/javalib/Makefile.conf
index a34afab69c..3f2dd15675 100644
--- a/usr/src/lib/libslp/javalib/Makefile.conf
+++ b/usr/src/lib/libslp/javalib/Makefile.conf
@@ -56,7 +56,7 @@ SLPCLASSPATH=$(CLASSES):$(SLPJAVASRC)
 
 # javac flags
 JFLAGS= -O -d $(CLASSES) -sourcepath $(SLPCLASSPATH)
-JFLAGS += -source 7 -target 7 -Xlint:-options
+JFLAGS += -source 8 -target 8 -Xlint:none
 
 # The default make rule for Java files
 COMPILE.java=$(JAVAC) $(JFLAGS)