Limit _XOPEN_SOURCE to Linux.

POSIX feature macros are a mess. Reportedly, FreeBSD also breaks with
_XOPEN_SOURCE, so try leaving it unset by default.

Update-Note: It's possible this will break yet another obscure UNIX.
Hopefully we can eventually find a combination that works?

Bug: 471
Change-Id: I103f8093110d343789b9c5a22eb056ab78d9cd14
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/51145
Reviewed-by: Adam Langley <agl@google.com>

CMakeLists.txt

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