diff --git a/CMakeLists.txt b/CMakeLists.txt
index 923358d..98aa02c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -67,12 +67,37 @@ if(cpu STREQUAL "powerpc")
 	set(cpu "ppc")
 elseif(cpu MATCHES "i.86")
 	set(cpu "x86")
+elseif(cpu MATCHES "[aA][mM][dD]64" OR cpu MATCHES "[xX]64")
+	set(cpu "x86_64")
 endif()
 
-if(MSVC AND CMAKE_CL_64)
-	set(cpu "amd64")
+# On Windows ${CMAKE_SYSTEM_PROCESSOR} is broken (always returns host CPU)
+# which has only been reported >7 years ago (https://cmake.org/pipermail/cmake-developers/2014-September/011405.html)
+# so obviously a fix is too much to ask. Here's the special case to make that wonderful platform work;
+# except if it's Windows for ARM(64) I guess, no idea how to detect that properly (I don't own such hardware).
+if(MSVC)
+	if(cpu MATCHES ".*[aA][rR][mM].*")
+		message(FATAL_ERROR "please fix this code to work for Windows on ARM and send a pull request")
+	endif()
+	if(CMAKE_CL_64)
+		set(cpu "x86_64")
+	else()
+		set(cpu "x86")
+	endif()
+elseif(DEFINED ENV{MINGW_CHOST})
+	# looks like it's broken in MinGW32 shells (or 32bit mingw-w64 shells) as well, this should help:
+	message(STATUS "MINGW_CHOST = $ENV{MINGW_CHOST}")
+	if($ENV{MINGW_CHOST} MATCHES "^i.86.*")
+		set(cpu "x86")
+	elseif($ENV{MINGW_CHOST} MATCHES "^x86_64.*")
+		set(cpu "x86_64")
+	else()
+		message(FATAL_ERROR "please fix this code to work for MINGW_CHOST = $ENV{MINGW_CHOST} and send a pull request!")
+	endif()
 endif()
 
+add_definitions(-DD3_ARCH="${cpu}" -DD3_SIZEOFPTR=${CMAKE_SIZEOF_VOID_P})
+
 # target os
 if(APPLE)
 	set(os "macosx")
@@ -80,6 +105,16 @@ else()
 	string(TOLOWER "${CMAKE_SYSTEM_NAME}" os)
 endif()
 
+add_definitions(-DD3_OSTYPE="${os}")
+
+message(STATUS "Setting -DD3_ARCH=\"${cpu}\" -DD3_SIZEOFPTR=${CMAKE_SIZEOF_VOID_P} -DD3_OSTYPE=\"${os}\" ")
+
+if(cpu MATCHES ".*64.*" AND NOT CMAKE_SIZEOF_VOID_P EQUAL 8)
+	# tough luck if some CPU architecture has "64" in its name but uses 32bit pointers
+	message(SEND_ERROR "CMake thinks sizeof(void*) == 4, but the target CPU looks like a 64bit CPU!")
+	message(FATAL_ERROR "If you're building in a 32bit chroot on a 64bit host, switch to it with 'linux32 chroot' or at least call cmake with linux32 (or your OSs equivalent)!")
+endif()
+
 # build type
 if(NOT CMAKE_BUILD_TYPE)
 	set(CMAKE_BUILD_TYPE "RelWithDebInfo")