spinel: Provide alternate implementaiton of strnlen() (#282)

Per isue #279, we cannot rely upon the existance of `strnlen()`.
This change adds a small replacement funciton to replace it if
it is missing.

configure.ac

@@ -409,6 +409,11 @@ AM_CONDITIONAL([OPENTHREAD_ENABLE_NCP],      [test "${enable_ncp}" != "no"])
 AM_CONDITIONAL([OPENTHREAD_ENABLE_NCP_SPI],  [test "${enable_ncp}"  = "spi"])
 AM_CONDITIONAL([OPENTHREAD_ENABLE_NCP_UART], [test "${enable_ncp}"  = "uart"])
 
+if test "${enable_ncp}" != "no"
+then
+    AC_CHECK_FUNCS([strnlen])
+fi
+
 #
 # Examples
 #

src/ncp/spinel.c

@@ -71,7 +71,7 @@
 #endif
 
 #if defined(errno) && SPINEL_PLATFORM_DOESNT_IMPLEMENT_ERRNO_VAR
-#error SPINEL_PLATFORM_DOESNT_IMPLEMENT_ERRNO_VAR is set but errno is already defined.
+#error "SPINEL_PLATFORM_DOESNT_IMPLEMENT_ERRNO_VAR is set but errno is already defined."
 #endif
 
 // Work-around for platforms that don't implement the `errno` variable.
@@ -90,6 +90,20 @@ static int spinel_errno_workaround_;
 #endif // else SPINEL_PLATFORM_DOESNT_IMPLEMENT_FPRINTF
 #endif
 
+#if !HAVE_STRNLEN
+// Provide a working strnlen if the platform doesn't have one.
+static size_t spinel_strnlen_(const char *s, size_t maxlen)
+{
+    size_t ret;
+    for (ret = 0; (ret < maxlen) && (s[ret] != 0); ret++)
+    {
+        // Empty loop.
+    }
+    return ret;
+}
+#define strnlen spinel_strnlen_
+#endif
+
 #ifndef require_action
 #define require_action(c, l, a) \
     do { if (!(c)) { \