https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=259334

--- ../../src/webrtc/src/third_party/boringssl/src/util/generate_build_files.py.orig	2022-05-04 15:47:04 UTC
+++ ../../src/webrtc/src/third_party/boringssl/src/util/generate_build_files.py
@@ -447,7 +447,7 @@ if(NOT WIN32)
 
 # pthread_rwlock_t requires a feature flag.
 if(NOT WIN32)
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D_XOPEN_SOURCE=700")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ")
 endif()
 
 if(WIN32)
--- ../../src/webrtc/src/third_party/boringssl/src/util/BUILD.toplevel.orig	2022-05-04 15:47:04 UTC
+++ ../../src/webrtc/src/third_party/boringssl/src/util/BUILD.toplevel
@@ -95,7 +95,7 @@ linux_copts = posix_copts + [
     # This is needed on Linux systems (at least) to get rwlock in pthread, but
     # it should not be set on Apple platforms, where it instead disables APIs
     # we use. See compat(5) and sys/cdefs.h.
-    "-D_XOPEN_SOURCE=700",
+    "",
 ]
 
 boringssl_copts = select({
--- ../../src/webrtc/src/third_party/boringssl/src/CMakeLists.txt.orig	2022-05-04 15:46:59 UTC
+++ ../../src/webrtc/src/third_party/boringssl/src/CMakeLists.txt
@@ -262,7 +262,7 @@ if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
 # 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 "${CMAKE_C_FLAGS} -D_XOPEN_SOURCE=700")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ")
 endif()
 
 if(FUZZ)
