external/boringssl: Sync to f21650709a6f76e829ddcc77fe221c9d6a5c12de.

This includes the following changes:

https://boringssl.googlesource.com/boringssl/+log/348f0d8db9c2a0eca0503ba654020209c579d552..f21650709a6f76e829ddcc77fe221c9d6a5c12de

Test: BoringSSL CTS Presubmits.
Change-Id: Ie6e99c3315c552068b5ea57e31b1af7ff94f9b0f
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index dc6e0d2..c32b707 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -125,6 +125,9 @@
   set(CMAKE_C_FLAGS   "-Wall -WX ${MSVC_DISABLED_WARNINGS_STR} ${MSVC_LEVEL4_WARNINGS_STR}")
   set(CMAKE_CXX_FLAGS "-Wall -WX ${MSVC_DISABLED_WARNINGS_STR} ${MSVC_LEVEL4_WARNINGS_STR}")
   set(CMAKE_ASM_NASM_FLAGS "-g cv8")
+endif()
+
+if(WIN32)
   add_definitions(-D_HAS_EXCEPTIONS=0)
   add_definitions(-DWIN32_LEAN_AND_MEAN)
   add_definitions(-DNOMINMAX)