Request C11 in CMake (but only for tests)

Set the C_STANDARD property on the mbedtls_test target to 11.
This requests C11 for the tests only.

If C11 is not supported the build will not fail, since
C_STANDARD_REQUIRED is not set, and memory poisoning will be disabled
by a preprocessor check on __STDC_VERSION__.

Additionally, reintroduce previous C99 enforcement on the rest of the
library.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 210cc38..0cb208e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -177,6 +177,9 @@
 
 include(CheckCCompilerFlag)
 
+set(CMAKE_C_EXTENSIONS OFF)
+set(CMAKE_C_STANDARD 99)
+
 if(CMAKE_COMPILER_IS_GNU)
     # some warnings we want are not available with old GCC versions
     # note: starting with CMake 2.8 we could use CMAKE_C_COMPILER_VERSION
@@ -294,6 +297,8 @@
         PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/tests/include
         PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/include
         PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/library)
+    # Request C11, needed for memory poisoning tests
+    set_target_properties(mbedtls_test PROPERTIES C_STANDARD 11)
 
     file(GLOB MBEDTLS_TEST_HELPER_FILES
          ${CMAKE_CURRENT_SOURCE_DIR}/tests/src/test_helpers/*.c)