CMake: Fix architecture detection on MSVC

CMAKE_SYSTEM_ARCHICTETURE always returns the *host* not target arch
when using the MSVC generators. (CMake bugs 15170 and 14342.)

CMakeLists.txt

@@ -2,6 +2,23 @@
 # dependent libraries.
 cmake_minimum_required(VERSION 2.8.11)
 
+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()
+
 project(citra)
 
 if(NOT EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/.git/hooks/pre-commit)
@@ -10,12 +27,21 @@ if(NOT EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/.git/hooks/pre-commit)
         DESTINATION ${CMAKE_CURRENT_SOURCE_DIR}/.git/hooks)
 endif()
 
-# Platform-agnostic definition to check if we are on x86_64
-if(${CMAKE_SYSTEM_PROCESSOR} MATCHES "[xX]86_64" OR
-   ${CMAKE_SYSTEM_PROCESSOR} MATCHES "[aA][mM][dD]64")
-    set(ARCHITECTURE_x86_64 1)
-    add_definitions(-DARCHITECTURE_x86_64=1)
+if (MSVC)
+    detect_architecture("_M_AMD64" x86_64)
+    detect_architecture("_M_IX86" x86)
+    detect_architecture("_M_ARM" ARM)
+else()
+    detect_architecture("__x86_64__" x86_64)
+    detect_architecture("__i386__" x86)
+    detect_architecture("__arm__" ARM)
+endif()
+if (NOT DEFINED ARCHITECTURE)
+    set(ARCHITECTURE "GENERIC")
+    set(ARCHITECTURE_GENERIC 1)
+    add_definitions(-DARCHITECTURE_GENERIC=1)
 endif()
+message(STATUS "Target architecture: ${ARCHITECTURE}")
 
 if (NOT MSVC)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wno-attributes -pthread")