Makefile: add a comment about JCVALID

In 857d87f, we changed to runtime resolution,
explaining why in a script comment, not only in a commit comment.

Makefile.config

@@ -160,6 +160,9 @@ JFLAGS     := -g -source 1.5 -target 1.5 -Xlint
 JAR        := jar
 
 # Check if the java compiler works
+#    Note that we defer JCVALID evaluation to runtime, 
+#    since $(JC) can be redefined later in a specific Makefile.config
+#    The core Makefile.config is then used as a Makefile.default
 JCVALID    = $(shell $(JC) -version >/dev/null 2>/dev/null && echo "yes")
 
 # Check whether setruid functionality can be used