cmake: Fix 64bit host CPU detection

In cases where we don't cross-compile, we might want to detect if a CPU
is 32bit or 64bit. CMake provides functionality for this case starting
in CMake 3.10. Let's use it.
Ubuntu 20.04 uses CMake 3.16. From the top of my head, this is the
oldest supported distribution. Debian buster ships with CMake 3.13.

Signed-off-by: Johannes Demel <[email protected]>

Author: jdemel

CMakeLists.txt

@@ -10,7 +10,9 @@
 ########################################################################
 # Project setup
 ########################################################################
-cmake_minimum_required(VERSION 3.8)
+# We use `IS_64BIT now: https://cmake.org/cmake/help/latest/command/cmake_host_system_information.html
+cmake_minimum_required(VERSION 3.10)
+
 set(CMAKE_BUILD_TYPE
     ${CMAKE_BUILD_TYPE}
     CACHE STRING "Choose build type: None Debug Release RelWithDebInfo MinSizeRel")

lib/CMakeLists.txt

@@ -255,17 +255,16 @@ endif()
 ########################################################################
 if(NOT CROSSCOMPILE_MULTILIB AND CPU_IS_x86)
     include(CheckTypeSize)
-    check_type_size("void*[8]" SIZEOF_CPU BUILTIN_TYPES_ONLY)
-    if(${SIZEOF_CPU} EQUAL 64)
+    cmake_host_system_information(RESULT ASSUME_64BIT_HOST QUERY IS_64BIT)
+    if(ASSUME_64BIT_HOST)
         overrule_arch(32 "CPU width is 64 bits")
-    endif()
-    if(${SIZEOF_CPU} EQUAL 32)
+    else()
         overrule_arch(64 "CPU width is 32 bits")
     endif()
 
     #MSVC 64 bit does not have MMX, overrule it
     if(MSVC)
-        if(${SIZEOF_CPU} EQUAL 64)
+        if(ASSUME_64BIT_HOST)
             overrule_arch(mmx "No MMX for Win64")
         endif()
         force_arch(sse "Built-in for MSVC > 2013")