In musl libc the return type of strerror_r(2) is int (XSI-compliant).
We define MUSL_LIBC in CXXFLAGS to skip the wrong #else implementation.

--- snapper/AppUtil.cc	2015-08-30 15:34:51.106531190 +0200
+++ snapper/AppUtil.cc	2015-08-30 15:35:41.996532260 +0200
@@ -209,7 +209,7 @@
     string
     stringerror(int errnum)
     {
-#if (_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600) && ! _GNU_SOURCE
+#if ((_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600) && ! _GNU_SOURCE) || MUSL_LIBC
 	char buf1[100];
 	if (strerror_r(errno, buf1, sizeof(buf1)-1) == 0)
 	    return string(buf1);