kconfig: Add symbols for GNU C Extensions

Add a symbol to enable GNU C Extensions. And a hidden option for
toolchains to signal GNU Extensions support.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>

CMakeLists.txt

@@ -971,6 +971,10 @@ if(NOT DEFINED CSTD)
   else()
     message(FATAL_ERROR "Unreachable code. Expected C standard to have been chosen.")
   endif()
+
+  if(CONFIG_GNU_C_EXTENSIONS)
+    string(REPLACE "c" "gnu" CSTD "${CSTD}")
+  endif()
 else()
   message(DEPRECATION
     "Global CSTD property is deprecated, see Kconfig.zephyr for C Standard options.")

Kconfig.zephyr

@@ -403,6 +403,19 @@ config STD_C23
 
 endchoice
 
+config TOOLCHAIN_SUPPORTS_GNU_EXTENSIONS
+	bool
+	default y if "$(ZEPHYR_TOOLCHAIN_VARIANT)" = "zephyr"
+	help
+	  Hidden option to signal that toolchain supports GNU Extensions.
+
+config GNU_C_EXTENSIONS
+	bool "GNU C Extensions"
+	depends on TOOLCHAIN_SUPPORTS_GNU_EXTENSIONS
+	help
+	  Enable GNU C Extensions. GNU C provides several language features
+	  not found in ISO standard C.
+
 config CODING_GUIDELINE_CHECK
 	bool "Enforce coding guideline rules"
 	help

cmake/toolchain/armclang/Kconfig

@@ -23,3 +23,7 @@ config ARMCLANG_STD_LIBC
 	  not provided by ARM Compiler standard libraries.
 
 endchoice
+
+config TOOLCHAIN_ARMCLANG_SUPPORTS_GNU_EXTENSIONS
+	def_bool y
+	select TOOLCHAIN_SUPPORTS_GNU_EXTENSIONS

cmake/toolchain/cross-compile/Kconfig

@@ -7,3 +7,10 @@ config TOOLCHAIN_CROSS_COMPILE_SUPPORTS_THREAD_LOCAL_STORAGE
 	help
 	  Set this if the cross-compile toolchain being used for the build
 	  supports thread local storage.
+
+config TOOLCHAIN_CROSS_COMPILE_SUPPORTS_GNU_EXTENSIONS
+	bool "Cross-compile toolchain supports GNU Extensions"
+	select TOOLCHAIN_SUPPORTS_GNU_EXTENSIONS
+	help
+	  Set this if the cross-compile toolchain being used for the build
+	  supports GNU Extensions.

cmake/toolchain/gnuarmemb/Kconfig

@@ -7,3 +7,7 @@ config TOOLCHAIN_GNUARMEMB
 	def_bool y
 	select HAS_NEWLIB_LIBC_NANO
 	select NEWLIB_LIBC_SUPPORTED
+
+config TOOLCHAIN_GNUARMEMB_SUPPORTS_GNU_EXTENSIONS
+	def_bool y
+	select TOOLCHAIN_SUPPORTS_GNU_EXTENSIONS

cmake/toolchain/host/Kconfig

@@ -0,0 +1,6 @@
+# Copyright (c) 2024 Basalte bv
+# SPDX-License-Identifier: Apache-2.0
+
+config TOOLCHAIN_HOST_SUPPORTS_GNU_EXTENSIONS
+	def_bool y
+	select TOOLCHAIN_SUPPORTS_GNU_EXTENSIONS

cmake/toolchain/llvm/Kconfig

@@ -22,3 +22,7 @@ config TOOLCHAIN_LLVM_SUPPORTS_THREAD_LOCAL_STORAGE
 	depends on RISCV
 	def_bool y
 	select TOOLCHAIN_SUPPORTS_THREAD_LOCAL_STORAGE
+
+config TOOLCHAIN_LLVM_SUPPORTS_GNU_EXTENSIONS
+	def_bool y
+	select TOOLCHAIN_SUPPORTS_GNU_EXTENSIONS