configure.ac: add option --coverage to CXXFLAGS (to avoid error error 'Hidden symbol atexit is referenced by DSO' when using libtool with gcov)

configure.ac

@@ -1342,6 +1342,7 @@ AC_SUBST(COVERAGE, $enable_coverage)
 AM_CONDITIONAL(STARPU_COVERAGE_ENABLED, [test "x$enable_coverage" = "xyes"])
 if test x$enable_coverage = xyes; then
 	CFLAGS="${CFLAGS} --coverage"
+	CXXFLAGS="${CXXFLAGS} --coverage"
 	LDFLAGS="${LDFLAGS} --coverage"
 fi