fpmcu: Change method of selecting C standard

Zephyr PR #71024[1] deprecated CSTD global variable for selecting C
standard. Appropriate Kconfig options should be used instead.

[1] zephyrproject-rtos/zephyr#71024

BUG=b:321092852
TEST=zmake build bloonchipper

Change-Id: I3debb2d9ffc7295ddfbbbd7a84dda51db3fd90a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/5498197
Tested-by: Patryk Duda <[email protected]>
Commit-Queue: Patryk Duda <[email protected]>
Code-Coverage: Zoss <[email protected]>
Reviewed-by: Tom Hughes <[email protected]>

Author: duda-patryk

third_party/boringssl/CMakeLists.txt

@@ -75,12 +75,8 @@ zephyr_library_link_libraries(crypto)
 #
 # -Wno-return-type is needed to ignore non-void functions that calls abort() and
 # don't have return statement (e.g. aes_hw_set_encrypt_key()).
-#
-# -std=c11 is necessary because BoringSSL sources use features available in
-# C11, e.g. static_assert().
 set(BORINGSSL_COMPILE_OPTIONS -D_POSIX_C_SOURCE=200809L
-	-Wno-implicit-function-declaration -fno-common -Wno-return-type
-	-std=c11)
+	-Wno-implicit-function-declaration -fno-common -Wno-return-type)
 
 # Create interface library that specifies compile options required by
 # BoringSSL. This is the same trick used by zephyr_library_compile_options()

third_party/boringssl/Kconfig

@@ -6,6 +6,7 @@ config BORINGSSL_CRYPTO
 	bool "BoringSSL support"
 	depends on COMMON_LIBC_MALLOC_ARENA_SIZE >= 2048 || \
 		   NEWLIB_LIBC_MIN_REQUIRED_HEAP_SIZE >= 2048
+	select REQUIRES_STD_C11
 	help
 	  This option enables BoringSSL cryptography library.
 	  The library is provided as external module.

zephyr/program/fpmcu/CMakeLists.txt

@@ -4,11 +4,6 @@
 
 cmake_minimum_required(VERSION 3.13.1)
 
-# Set C standard to C11. If not set, Zephyr uses C99 by default.
-# This property must be set before 'find_package' below, so Zephyr's top
-# CMakeLists.txt can set up everything properly.
-set_property(GLOBAL PROPERTY CSTD c11)
-
 find_package(Zephyr REQUIRED HINTS "${ZEPHYR_BASE}")
 
 # USAGE FAULT on int overflow

zephyr/program/fpmcu/prj.conf

@@ -10,6 +10,11 @@
 # check_init_priorities.py.
 CONFIG_CHECK_INIT_PRIORITIES=n
 
+# Use C11 standard to have _Static_assert() compiler builtin.
+# Zephyr will use it for BUILD_ASSERT() implementation (instead of silently
+# ignoring them).
+CONFIG_STD_C11=y
+
 # Power Management
 CONFIG_PM=y
 CONFIG_PM_DEVICE=y