Make ARCH variable a CACHE variable

cmake/c_check.cmake

@@ -61,7 +61,8 @@ endif ()
 # CMAKE_HOST_SYSTEM_PROCESSOR - The name of the CPU CMake is running on.
 #
 # TODO: CMAKE_SYSTEM_PROCESSOR doesn't seem to be correct - instead get it from the compiler a la c_check
-set(ARCH ${CMAKE_SYSTEM_PROCESSOR})
+set(ARCH ${CMAKE_SYSTEM_PROCESSOR} CACHE STRING "Target Architecture")
+
 if (${ARCH} STREQUAL "AMD64")
   set(ARCH "x86_64")
 endif ()