IAR: Enable extensions by default

Set the variable added by commit a2112257 (Add infrastructure to use
language extensions without any standard, 2017-06-29) for IAR.  This was
accidentally left out of commit d8e6cd9e (IAR: Improve support for IAR
ARM Compiler, 2017-06-15).

Modules/Compiler/IAR-C.cmake

@@ -5,6 +5,9 @@ include(Compiler/CMakeCommonCompilerMacros)
 
 # The toolchains for ARM and AVR are quite different:
 if("${CMAKE_C_COMPILER_ARCHITECTURE_ID}" STREQUAL "ARM")
+
+  set(CMAKE_C_EXTENSION_COMPILE_OPTION -e)
+
   set(CMAKE_C90_STANDARD_COMPILE_OPTION "")
   set(CMAKE_C90_EXTENSION_COMPILE_OPTION -e)
 

Modules/Compiler/IAR-CXX.cmake

@@ -17,6 +17,8 @@ if("${CMAKE_CXX_COMPILER_ARCHITECTURE_ID}" STREQUAL "ARM")
     endif()
   endif()
 
+  set(CMAKE_CXX_EXTENSION_COMPILE_OPTION -e)
+
   set(CMAKE_CXX98_STANDARD_COMPILE_OPTION "")
   set(CMAKE_CXX98_EXTENSION_COMPILE_OPTION -e)