In musl libc stdout and stderr are macros, thus we need to
undefine them to be able to use SystemCmd::stdout() and ::stderr()
members without renaming them - which may be a better choice.

--- a/snapper/SystemCmd.h	2015-08-30 10:59:53.740184303 +0200
+++ b/snapper/SystemCmd.h	2015-08-30 11:02:34.229187677 +0200
@@ -31,6 +31,8 @@
 #include <list>
 #include <boost/noncopyable.hpp>
 
+#undef stdout
+#undef stderr
 
 namespace snapper
 {