Fixed coverage in Makefile.

tests/rules.mk

 TESTS=$(wildcard tests/test_*.py)
 COVERAGE_OUTPUT_DIR := coverage
-OMIT := --omit /usr/share/pyshared/*
+OMIT := /usr/share/pyshared/*
+RM=rm -rf
 
 ifeq ($(findstring python-coverage,$(wildcard /usr/bin/*)), python-coverage)
 COVERAGE=/usr/bin/python-coverage
@@ -12,15 +13,28 @@ endif
 
 test: $(TESTS) build
 
+ifeq ($(findstring python-coverage,$(wildcard /usr/bin/*)), python-coverage)
+coverage: ${COVERAGE} build
+	${COVERAGE} erase
+	${RM} ${COVERAGE_OUTPUT_DIR}/*
+	mkdir ${COVERAGE_OUTPUT_DIR} 2>/dev/null || true
+	for t in ${TESTS}; do \
+		echo $$t; \
+		${COVERAGE} -x test.py $$t; \
+		${COVERAGE} combine; \
+	done
+	${COVERAGE} html --omit=${OMIT} -d ${COVERAGE_OUTPUT_DIR}
+else
 coverage: ${COVERAGE} build
 	mkdir ${COVERAGE_OUTPUT_DIR} 2>/dev/null || true
 	${COVERAGE} erase
 	for t in ${TESTS}; do \
 		echo $$t; \
-		${COVERAGE} ${OMIT} -x test.py $$t; \
-		${COVERAGE} ${OMIT} -c; \
+		${COVERAGE} --omit ${OMIT} -x test.py $$t; \
+		${COVERAGE} --omit ${OMIT} -c; \
 	done
-	${COVERAGE} html ${OMIT} --dir ${COVERAGE_OUTPUT_DIR}
+	${COVERAGE} html --omit ${OMIT} --dir ${COVERAGE_OUTPUT_DIR}
+endif
 
 ${COVERAGE}:
 	@echo "Install package 'python-coverage' to generate a coverage report."