build-sys: check for strndupa

strndupa is a GNU extension, therefore it's not available
for all libc libraries (musl).

This patch is based on the one proposed by Emil Renner Berthing for
systemd [1].

[1] http://lists.freedesktop.org/archives/systemd-devel/2014-September/023190.html

Signed-off-by: Romain Naour <romain.naour@openwide.fr>

configure.ac

@@ -66,8 +66,9 @@ AC_C_INLINE
 AC_TYPE_MODE_T
 AC_TYPE_PID_T
 AC_CHECK_MEMBERS([struct stat.st_rdev])
-AC_CHECK_DECLS([getrandom, gettid, name_to_handle_at, accept4, mkostemp], [], [], [[#include <sys/types.h>
+AC_CHECK_DECLS([getrandom, gettid, name_to_handle_at, accept4, mkostemp, strndupa], [], [], [[#include <sys/types.h>
 #include <unistd.h>
+#include <string.h>
 #include <sys/mount.h>
 #include <fcntl.h>
 #include <sys/socket.h>

src/shared/missing.h

@@ -158,3 +158,14 @@ static inline int name_to_handle_at(int fd, const char *name, struct file_handle
 #ifndef AT_EMPTY_PATH
 #define AT_EMPTY_PATH 0x1000
 #endif
+
+#if !HAVE_DECL_STRNDUPA
+#define strndupa(s, n) \
+  ({ \
+    const char *__old = (s); \
+    size_t __len = strnlen(__old, (n)); \
+    char *__new = (char *)alloca(__len + 1); \
+    __new[__len] = '\0'; \
+    (char *)memcpy(__new, __old, __len); \
+  })
+#endif

src/shared/selinux-util.c

@@ -31,6 +31,7 @@
 #include "strv.h"
 #include "path-util.h"
 #include "selinux-util.h"
+#include "missing.h"
 
 #ifdef HAVE_SELINUX
 DEFINE_TRIVIAL_CLEANUP_FUNC(security_context_t, freecon);