Export the values of the following standard compiler and compiler flag
variables in the main makefile, avoiding the need to pass through each
one to child make processes manually:
CC
CXX
LD
JAVAC
JAVACC
Since JAVAC is manually exported as "$(JAVAC) $(JFLAGS)" by the main
makefile, additionally separate JAVAC and JFLAGS into separate variables
from the perspective of child make processes.