CMake: Add autodetection for 32-bit x86 CRC assembly usage

CMakeLists.txt

@@ -243,7 +243,24 @@ add_compile_definitions(
 
 # Support 32-bit x86 assembly files.
 if(NOT MSVC)
-    option(XZ_ASM_I386 "Enable 32-bit x86 assembly code" OFF)
+    # Autodetection isn't so simple as in configure.ac because with
+    # CMake we don't have as reliable variable as Autoconf's $host_cpu.
+    # CMAKE_SYSTEM_PROCESSOR could indicate x86-64 even if building
+    # for 32-bit x86 because one doesn't necessarily use a toolchain
+    # file when building 32-bit executables on 64-bit system. Also,
+    # the strings that mean 32-bit or 64-bit x86 aren't standardized
+    # in CMake... So it's safer to ask the compiler when we seem to
+    # be building for a compatible platform.
+    if(MINGW OR CYGWIN OR CMAKE_SYSTEM_NAME MATCHES
+            "^FreeBSD$|^GNU$|^Linux$|^MirBSD$|^NetBSD$|^OpenBSD$")
+        check_symbol_exists("__i386__" "" ASM_I386_DEFAULT)
+    else()
+        set(ASM_I386_DEFAULT OFF)
+    endif()
+
+    option(XZ_ASM_I386 "Enable 32-bit x86 assembly code"
+           "${ASM_I386_DEFAULT}")
+
     if(XZ_ASM_I386)
         enable_language(ASM)
     endif()