Update prism launch script on macOS.

Only use /usr/libexec/java_home if java is in /usr/bin.
That is the problematic case for passing DYLD_LIBRARY_PATH
and some Java installs do not work with /usr/libexec/java_home.

prism/src/bin/prism.darwin

@@ -7,12 +7,13 @@ PRISM_DIR=/home/luser/prism
 
 # Command to launch Java
 if [ "$PRISM_JAVA" = "" ]; then
+	PRISM_JAVA=java
 	# On OS X, we want to avoiding calling java from the /usr/bin link
 	# since it causes problems with dynamic linking (DYLD_LIBRARY_PATH)
-	if [ -x /usr/libexec/java_home ]; then
-		PRISM_JAVA=`/usr/libexec/java_home`"/bin/java"
-	else
-		PRISM_JAVA=java
+	if $(which java | grep -q '^/usr/bin'); then
+		if [ -x /usr/libexec/java_home ]; then
+			PRISM_JAVA=`/usr/libexec/java_home`"/bin/java"
+		fi
 	fi
 fi