testing: simplify Intel compiler ID check

Testing/External/CMakeLists.txt

@@ -42,7 +42,7 @@ set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} ${VTK_REQUIRED_SHARE
 set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} ${VTK_REQUIRED_MODULE_LINKER_FLAGS}")
 
 # This is a cross-platform project so we cannot use the MS _s API.
-if(WIN32 AND "${CMAKE_C_COMPILER_ID}" MATCHES "^(Intel)$")
+if(WIN32 AND "x${CMAKE_C_COMPILER_ID}" STREQUAL "xIntel")
   set(_INTEL_WINDOWS 1)
 endif()
 if(MSVC OR _INTEL_WINDOWS)