Test cpp_dummy_build in pedantic mode

In public headers, we want to avoid things that are not standard C++,
including features that GCC and Clang support as extensions, such as
flexible array members. So compile with `-pedantic`.

Non-regression for https://github.com/Mbed-TLS/mbedtls/issues/9020.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 10f4f53..d02b84e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -281,6 +281,15 @@
     endif(CMAKE_COMPILER_IS_IAR)
 endif(MBEDTLS_FATAL_WARNINGS)
 
+if(CMAKE_BUILD_TYPE STREQUAL "Check" AND TEST_CPP)
+    set(CMAKE_CXX_STANDARD 11)
+    set(CMAKE_CXX_STANDARD_REQUIRED ON)
+    set(CMAKE_CXX_EXTENSIONS OFF)
+    if(CMAKE_COMPILER_IS_CLANG OR CMAKE_COMPILER_IS_GNU)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic")
+    endif()
+endif()
+
 if(CMAKE_BUILD_TYPE STREQUAL "Coverage")
     if(CMAKE_COMPILER_IS_GNU OR CMAKE_COMPILER_IS_CLANG)
         set(CMAKE_SHARED_LINKER_FLAGS "--coverage")