cmake: toolchain: generalize exclusion of CXX options

-Wold-style-definition is not a supported option for C++ builds.  To
prevent it being passed:
* the list of compiler flags to be excluded from C++ builds is moved
  to be toolchain-specific;
* -Wold-style-definition is added to that list for gcc and clang;
* -Wold-style-definition is moved from zephyr_compiler_options to
  zephyr_cc_option so the option checking code is executed for it.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>

cmake/compiler/clang/target_warnings.cmake

@@ -27,9 +27,9 @@ macro(toolchain_cc_warning_dw_1)
     -Wno-unused-parameter
     -Wmissing-declarations
     -Wmissing-format-attribute
-    -Wold-style-definition
     )
   zephyr_cc_option(
+    -Wold-style-definition
     -Wmissing-prototypes
     -Wmissing-include-dirs
     -Wunused-but-set-variable
@@ -115,3 +115,10 @@ endmacro()
 macro(toolchain_cc_cpp_warning_error_misra_sane dest_var_name)
   set_ifndef(${dest_var_name} "-Werror=vla")
 endmacro()
+
+# List the warnings that are not supported for C++ compilations
+
+list(APPEND CXX_EXCLUDED_OPTIONS
+  -Werror=implicit-int
+  -Wold-style-definition
+  )

cmake/compiler/gcc/target_warnings.cmake

@@ -34,9 +34,9 @@ macro(toolchain_cc_warning_dw_1)
     -Wno-unused-parameter
     -Wmissing-declarations
     -Wmissing-format-attribute
-    -Wold-style-definition
     )
   zephyr_cc_option(
+    -Wold-style-definition
     -Wmissing-prototypes
     -Wmissing-include-dirs
     -Wunused-but-set-variable
@@ -108,3 +108,10 @@ endmacro()
 macro(toolchain_cc_cpp_warning_error_misra_sane dest_var_name)
   set_ifndef(${dest_var_name} "-Werror=vla")
 endmacro()
+
+# List the warnings that are not supported for C++ compilations
+
+list(APPEND CXX_EXCLUDED_OPTIONS
+  -Werror=implicit-int
+  -Wold-style-definition
+  )

cmake/extensions.cmake

@@ -851,10 +851,10 @@ function(zephyr_check_compiler_flag lang option check)
 endfunction()
 
 function(zephyr_check_compiler_flag_hardcoded lang option check exists)
-  # -Werror=implicit-int is not supported for CXX and we are not able
-  # to automatically test for it because it produces a warning
-  # instead of an error during the test.
-  if((${lang} STREQUAL CXX) AND ("${option}" STREQUAL -Werror=implicit-int))
+  # Various flags that are not supported for CXX may not be testable
+  # because they would produce a warning instead of an error during
+  # the test.  Exclude them by toolchain-specific blacklist.
+  if((${lang} STREQUAL CXX) AND ("${option}" IN_LIST CXX_EXCLUDED_OPTIONS))
     set(check 0 PARENT_SCOPE)
     set(exists 1 PARENT_SCOPE)
   else()