string_util.c: provide fallback strndup() implementation

This patch also adds extern "C" { } wrapper around string_util.h
to allow its usage in C++ code

configure.ac

@@ -135,6 +135,8 @@ AC_SEARCH_LIBS([gethostbyname], [nsl])
 
 AC_CHECK_FUNCS(pipe2 accept4 eventfd)
 
+AC_CHECK_FUNCS(strndup)
+
 AC_SEARCH_LIBS([exp], [m],,
 	[AC_MSG_ERROR([exp() not found])])