tuklib_integer: Autodetect support for unaligned access on ARM.

The result is used as the default for --enable-unaligned-access.
The test should work with GCC and Clang.

m4/tuklib_integer.m4

@@ -58,6 +58,17 @@ if test "x$enable_unaligned_access" = xauto ; then
 		i?86|x86_64|powerpc|powerpc64)
 			enable_unaligned_access=yes
 			;;
+		arm*|aarch64*)
+			# On 32-bit and 64-bit ARM, GCC and Clang
+			# #define __ARM_FEATURE_UNALIGNED if
+			# unaligned access is supported.
+			AC_COMPILE_IFELSE([AC_LANG_SOURCE([
+#ifndef __ARM_FEATURE_UNALIGNED
+compile error
+#endif
+int main(void) { return 0; }
+])], [enable_unaligned_access=yes], [enable_unaligned_access=no])
+			;;
 		*)
 			enable_unaligned_access=no
 			;;