diff --git a/CMakeLists.txt b/CMakeLists.txt
index f74e233..36bc8ec 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -257,10 +257,11 @@
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c11")
 endif()
 
-# pthread_rwlock_t on Linux requires a feature flag. However, it should not be
-# set on Apple platforms, where it instead disables APIs we use. See compat(5)
-# and sys/cdefs.h.
-if(NOT WIN32 AND NOT APPLE)
+# pthread_rwlock_t on Linux requires a feature flag. We limit this to Linux
+# because, on Apple platforms, it instead disables APIs we use. See compat(5)
+# and sys/cdefs.h. Reportedly, FreeBSD also breaks when this is set. See
+# https://crbug.com/boringssl/471.
+if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D_XOPEN_SOURCE=700")
 endif()