Avoid -Woverride in clang <12.0.0

cmake/GtsamBuildTypes.cmake

@@ -104,7 +104,16 @@ if(MSVC)
   set(GTSAM_COMPILE_OPTIONS_PRIVATE_TIMING          /MD /O2  CACHE STRING "(User editable) Private compiler flags for Timing configuration.")
 else()
   # Common to all configurations, next for each configuration:
-  
+
+  message (STATUS "CMAKE_CXX_COMPILER_VERSION: ${CMAKE_CXX_COMPILER_VERSION}")
+
+  if (
+      ((CMAKE_CXX_COMPILER_ID MATCHES "Clang") AND (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 12.0.0)) OR
+      (CMAKE_CXX_COMPILER_ID MATCHES "GNU")
+     )
+    set(flag_override_ -Wsuggest-override -Werror=suggest-override)
+  endif()
+
   set(GTSAM_COMPILE_OPTIONS_PRIVATE_COMMON
     -Wall                                          # Enable common warnings
     -fPIC                                          # ensure proper code generation for shared libraries
@@ -112,7 +121,7 @@ else()
     $<$<CXX_COMPILER_ID:Clang>:-Wreturn-stack-address   -Werror=return-stack-address>  # Error: return local address
     -Wreturn-type  -Werror=return-type             # Error on missing return()
     -Wformat -Werror=format-security               # Error on wrong printf() arguments
-    $<$<COMPILE_LANGUAGE:CXX>:-Wsuggest-override -Werror=suggest-override>      # Enforce the use of the override keyword
+    $<$<COMPILE_LANGUAGE:CXX>:${flag_override_}>   # Enforce the use of the override keyword
     #
     CACHE STRING "(User editable) Private compiler flags for all configurations.")
   set(GTSAM_COMPILE_OPTIONS_PRIVATE_DEBUG           -g -fno-inline  CACHE STRING "(User editable) Private compiler flags for Debug configuration.")