third_party: boringssl: Set _POSIX_C_SOURCE

BoringSSL sources require strdup() function which is defined only
when _POSIX_VISIBLE is at least 200809

BUG=b:321092852
TEST=zmake build -a
TEST=Run Gitlab CI locally

Change-Id: Idedc6ec0b26b9565c1c25f389fed184598c8ed07
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/5424639
Reviewed-by: Tom Hughes <tomhughes@chromium.org>
Tested-by: Patryk Duda <patrykd@google.com>
Code-Coverage: Zoss <zoss-cl-coverage@prod.google.com>
Commit-Queue: Tom Hughes <tomhughes@chromium.org>
(cherry picked from commit cfcca85636)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/5434153
Reviewed-by: Dawid Niedźwiecki <dawidn@google.com>
Commit-Queue: Patryk Duda <patrykd@google.com>

third_party/boringssl/CMakeLists.txt

@@ -70,6 +70,9 @@ zephyr_library_sources_ifdef(CONFIG_BORINGSSL_HARDWARE_ENTROPY
 zephyr_library_compile_options(-std=c11)
 
 # Add compile options required by BoringSSL.
+# -D_POSIX_C_SOURCE=200809L is needed because BoringSSL uses strdup() function
+# is guarded by __POSIX_VISIBLE >=200809 in Newlib's string.h file.
+#
 # -Wno-implicit-function-declaration is needed because BoringSSL sources use
 # functions not defined in Zephyr: fflush(), getenv(), sscanf(). We won't get
 # any linker issues because code that uses these functions is unused and will
@@ -81,7 +84,8 @@ zephyr_library_compile_options(-std=c11)
 #
 # -std=c11 is necessary because BoringSSL sources use features available in
 # C11, e.g. static_assert().
-set(BORINGSSL_COMPILE_OPTIONS -Wno-implicit-function-declaration -fno-common -std=c11)
+set(BORINGSSL_COMPILE_OPTIONS -D_POSIX_C_SOURCE=200809L
+	-Wno-implicit-function-declaration -fno-common -std=c11)
 
 # Target 'fipsmodule' is a object library. It compiles source files, but
 # doesn't archive or link object files into library. Other targets should