Additional CMake flag for controlling C standard

Disable compiler-specific extensions (e.g. use --std=c11 instead
of --std=gnu11).

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index ec14024..0c207d1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -170,6 +170,7 @@
 
 include(CheckCCompilerFlag)
 
+set(CMAKE_C_EXTENSIONS OFF)
 set(CMAKE_C_STANDARD 99)
 
 if(CMAKE_COMPILER_IS_GNU)