kconfig: Add symbols for C standard

Add symbols to select a C standard. Next to an option, hidden symbols
are available to select the minimum.
This allows subsystems or modules to select the least compliant C
standard supported.

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

CMakeLists.txt

@@ -957,7 +957,24 @@ if(CONFIG_USERSPACE)
 endif()
 
 get_property(CSTD GLOBAL PROPERTY CSTD)
-set_ifndef(CSTD c99)
+if(NOT DEFINED CSTD)
+  if(CONFIG_STD_C23)
+    set(CSTD c2x)
+  elseif(CONFIG_STD_C17)
+    set(CSTD c17)
+  elseif(CONFIG_STD_C11)
+    set(CSTD c11)
+  elseif(CONFIG_STD_C99)
+    set(CSTD c99)
+  elseif(CONFIG_STD_C90)
+    set(CSTD c90)
+  else()
+    message(FATAL_ERROR "Unreachable code. Expected C standard to have been chosen.")
+  endif()
+else()
+  message(DEPRECATION
+    "Global CSTD property is deprecated, see Kconfig.zephyr for C Standard options.")
+endif()
 
 # @Intent: Obtain compiler specific flag for specifying the c standard
 zephyr_compile_options(

Kconfig.zephyr

@@ -338,6 +338,71 @@ endmenu
 
 menu "Compiler Options"
 
+config REQUIRES_STD_C99
+	bool
+	help
+	  Hidden option to select compiler support C99 standard or higher.
+
+config REQUIRES_STD_C11
+	bool
+	select REQUIRES_STD_C99
+	help
+	  Hidden option to select compiler support C11 standard or higher.
+
+config REQUIRES_STD_C17
+	bool
+	select REQUIRES_STD_C11
+	help
+	  Hidden option to select compiler support C17 standard or higher.
+
+config REQUIRES_STD_C23
+	bool
+	select REQUIRES_STD_C17
+	help
+	  Hidden option to select compiler support C23 standard or higher.
+
+choice STD_C
+	prompt "C Standard"
+	default STD_C23 if REQUIRES_STD_C23
+	default STD_C17 if REQUIRES_STD_C17
+	default STD_C11 if REQUIRES_STD_C11
+	default STD_C99
+	help
+	  C Standards.
+
+config STD_C90
+	bool "C90"
+	depends on !REQUIRES_STD_C99
+	help
+	  1989 C standard as completed in 1989 and ratified by ISO/IEC
+	  as ISO/IEC 9899:1990. This version is known as "ANSI C".
+
+config STD_C99
+	bool "C99"
+	depends on !REQUIRES_STD_C11
+	help
+	  1999 C standard.
+
+config STD_C11
+	bool "C11"
+	depends on !REQUIRES_STD_C17
+	help
+	  2011 C standard.
+
+config STD_C17
+	bool "C17"
+	depends on !REQUIRES_STD_C23
+	help
+	  2017 C standard, addresses defects in C11 without introducing
+	  new language features.
+
+config STD_C23
+	bool "C23"
+	help
+	  2023 C standard.
+
+endchoice
+
 config CODING_GUIDELINE_CHECK
 	bool "Enforce coding guideline rules"
 	help