coverage: Pass coverage flags in CXXFLAGS

Older versions of setuptools pass CFLAGS to the C++ compiler.
Newer versions of setuptools pass only CXXFLAGS to the C++ compiler.

Set the profiling compiler flags in both CFLAGS and CXXFLAGS for now, to
cope with both old and new versions of setuptools.

Signed-off-by: Matt Wozniski <godlygeek@gmail.com>

Author: godlygeek

Makefile

@@ -71,7 +71,7 @@ valgrind:  ## Run valgrind, with the correct configuration
 .PHONY: ccoverage
 ccoverage:  ## Run the test suite, with C++ code coverage
 	$(MAKE) clean
-	CFLAGS="$(CFLAGS) -O0 -pg --coverage" $(MAKE) build
+	CFLAGS="$(CFLAGS) -O0 -pg --coverage" CXXFLAGS="$(CXXFLAGS) -O0 -pg --coverage" $(MAKE) build
 	$(MAKE) check
 	gcov -i build/*/src/pystack/_pystack -i -d
 	lcov --capture --directory .  --output-file cppcoverage.lcov