Fix build on Unix systems without POSIX realtime extensions.

This allows BoringSSL to build on OpenBSD with gcc/g++ 4.9.2.

Change-Id: Icce23de87b0358a581124eb8cd37dc48a1f096c9
Reviewed-on: https://boringssl-review.googlesource.com/5401
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/tool/CMakeLists.txt b/tool/CMakeLists.txt
index 7907745..2d706ba 100644
--- a/tool/CMakeLists.txt
+++ b/tool/CMakeLists.txt
@@ -19,5 +19,10 @@
 if (APPLE OR WIN32 OR ANDROID)
   target_link_libraries(bssl ssl crypto)
 else()
-  target_link_libraries(bssl ssl crypto -lrt)
+  find_library(FOUND_LIBRT rt)
+  if (FOUND_LIBRT)
+    target_link_libraries(bssl ssl crypto -lrt)
+  else()
+    target_link_libraries(bssl ssl crypto)
+  endif()
 endif()