prng: support getrandom & getentropy

If the C library supports these random functions, use them directly
instead of reading the /dev/urandom file.  This makes life easier on
platforms that don't have /dev/urandom (like WASM).

configure.ac

@@ -272,6 +272,7 @@ AC_CHECK_HEADERS(m4_normalize([
   stdlib.h
   string.h
   sys/ioctl.h
+  sys/random.h
   sys/resource.h
   sys/socket.h
   sys/stat.h
@@ -304,6 +305,8 @@ AC_TYPE_UINTPTR_T
 
 # Checks for library functions.
 AC_CHECK_FUNCS(m4_normalize([
+  getentropy
+  getrandom
   gettimeofday
   posix_memalign
   cfmakeraw