win/build: detect architecture

third-party/cmake/TargetArch.cmake

@@ -0,0 +1,23 @@
+# Sets TARGET_ARCH to a normalized name (X86 or X86_64).
+# See https://github.com/axr/solar-cmake/blob/master/TargetArch.cmake
+include(CheckSymbolExists)
+
+# X86
+check_symbol_exists("_M_IX86" "" T_M_IX86)
+check_symbol_exists("__i386__" "" T_I386)
+if(T_M_IX86 OR T_I386)
+set(TARGET_ARCH "X86")
+  return()
+endif()
+
+# X86_64
+check_symbol_exists("_M_AMD64" "" T_M_AMD64)
+check_symbol_exists("__x86_64__" "" T_X86_64)
+check_symbol_exists("__amd64__" "" T_AMD64)
+
+if(T_M_AMD64 OR T_X86_64 OR T_AMD64)
+set(TARGET_ARCH "X86_64")
+  return()
+endif()
+
+message(FATAL_ERROR "Unknown target architecture")