For musl libc it is required to include <sys/reg.h> to
have __WORDSIZE defined to e.g. 32 for arm*-musl.

--- install/util.h	2015-06-15 12:27:21.000000000 +0200
+++ install/util.h	2015-09-02 09:07:57.321104222 +0200
@@ -33,6 +33,9 @@
 #include <signal.h>
 #include <sched.h>
 #include <limits.h>
+#if !defined(__GLIBC__)
+#include <sys/reg.h>
+#endif
 #include <sys/stat.h>
 #include <dirent.h>
 #include <sys/resource.h>