The only required bit from Autoconf 2.64 is AS_VAR_IF. Which is in fact
provided, but just in case there is now a fallback implementation in
m4/sb_autoconf_compat.m4.
Use ax_gcc_archflag.m4 to try to guess the most optimal -march value for
the target architecture, unless cross-compiling, or the value is set
explicitly with --with-gcc-arch.
Also, AArch64/ThunderX-specific updates to ax_gcc_archflag.m4.
In sb_concurrency_kit.m4, add --enable-lse to CK build flags if LSE
instructions are supported by the target.
