Correcting the build target coverage for enabled SSL (#1009)

* Exclude includes from /usr in coverage reporting

* Correct build target `coverage` for enabled ssl

`USE_SSL=1 make coverage` will now build the test binary with the
forwarded define HIREDIS_TEST_SSL. This avoids inconsistency between
built test binary and the testrunner `test.sh`.
This enables test coverage measurements for SSL too.

Makefile

@@ -73,7 +73,7 @@ USE_SSL?=0
 
 # This is required for test.c only
 ifeq ($(USE_SSL),1)
-  CFLAGS+=-DHIREDIS_TEST_SSL
+  export CFLAGS+=-DHIREDIS_TEST_SSL
 endif
 
 ifeq ($(uname_S),Linux)
@@ -299,12 +299,12 @@ gprof:
 	$(MAKE) CFLAGS="-pg" LDFLAGS="-pg"
 
 gcov:
-	$(MAKE) CFLAGS="-fprofile-arcs -ftest-coverage" LDFLAGS="-fprofile-arcs"
+	$(MAKE) CFLAGS+="-fprofile-arcs -ftest-coverage" LDFLAGS="-fprofile-arcs"
 
 coverage: gcov
 	make check
 	mkdir -p tmp/lcov
-	lcov -d . -c -o tmp/lcov/hiredis.info
+	lcov -d . -c --exclude '/usr*' -o tmp/lcov/hiredis.info
 	genhtml --legend -o tmp/lcov/report tmp/lcov/hiredis.info
 
 noopt: