CMakeLists: Clean up

Moves functions out of the main CMakeLists file into module files that
can just be included whenever necessary. This also uses the CMake
provided variables for enforcing compiler requirements.

CMakeModules/DetectArchitecture.cmake

@@ -0,0 +1,16 @@
+include(CheckSymbolExists)
+function(detect_architecture symbol arch)
+    if (NOT DEFINED ARCHITECTURE)
+        set(CMAKE_REQUIRED_QUIET 1)
+        check_symbol_exists("${symbol}" "" ARCHITECTURE_${arch})
+        unset(CMAKE_REQUIRED_QUIET)
+
+        # The output variable needs to be unique across invocations otherwise
+        # CMake's crazy scope rules will keep it defined
+        if (ARCHITECTURE_${arch})
+            set(ARCHITECTURE "${arch}" PARENT_SCOPE)
+            set(ARCHITECTURE_${arch} 1 PARENT_SCOPE)
+            add_definitions(-DARCHITECTURE_${arch}=1)
+        endif()
+    endif()
+endfunction()