Fix more problems with chpl-env-gen.h

Follow-up to commit Avoid problems with + in chpl-env-gen.h
Replace . with _ and don't make defines for CHPL_*_CC / CHPL_*_CXX

---
Signed-off-by: Michael Ferguson <mppf@users.noreply.github.com>

runtime/Makefile

@@ -59,8 +59,11 @@ $(CHPL_ENV_HEADER): $(CHPL_MAKE_HOME)/util/printchplenv $(CHPL_MAKE_HOME)/util/c
 	@echo "#ifndef _CHPL_ENV_GEN_H_" >> $(CHPL_ENV_HEADER)
 	@echo "#define _CHPL_ENV_GEN_H_" >> $(CHPL_ENV_HEADER)
 	@$(CHPL_MAKE_HOME)/util/printchplenv --runtime --no-tidy --anonymize --simple | \
-          grep -v _COMMAND_ | \
-          sed s/[-+\/]/_/g | \
+          grep -v CHPL_HOST_CC | \
+          grep -v CHPL_HOST_CXX | \
+          grep -v CHPL_TARGET_CC | \
+          grep -v CHPL_TARGET_CXX | \
+          sed s/[-+.\/]/_/g | \
           sed s/\=/' '/ | \
           awk '{ print "#define " $$1 "_" toupper($$2) }' >> $(CHPL_ENV_HEADER)
 	@echo "#endif /* _CHPL_ENV_GEN_H_ */" >> $(CHPL_ENV_HEADER)