Build: Require Boost.Asio to be v1.69.0+

configure.ac

@@ -32,7 +32,7 @@ AC_DEFINE_UNQUOTED([CONFIG_MIND_VOSCILLATOR_FREQ_MS],
     [Frequency of the mind virtual oscillator in milliseconds])
 
 m4_include([m4/ax_boost_base.m4])
-m4_include([m4/ax_boost_asio.m4])
+m4_include([m4/ax_boost_asio_1.69.0.m4])
 m4_include([m4/ac_prog_flex.m4])
 m4_include([m4/ac_prog_bison.m4])
 AC_PROG_CC
@@ -44,9 +44,10 @@ AC_PROG_YACC
 AS_IF([test -z "${LEX}" || test -z "${YACC}"], [
     AC_MSG_ERROR([LEX and YACC must both be available in PATH.])
 ])
-#AX_BOOST_BASE
-AX_BOOST_ASIO
-AS_IF([test "x${HAVE_BOOST_ASIO}" != "xyes"], [
+AX_BOOST_BASE([1.69.0], [], [AC_MSG_ERROR(m4_normalize([Boost v1.69.0 or higher
+    is required, because Boost.System is header-only from 1.69.0 onwards.]))])
+AX_BOOST_ASIO_gte_1_69_0
+AS_IF([test "x${HAVE_BOOST_ASIO}" == "x"], [
     AC_MSG_ERROR(m4_normalize([Boost.Asio must be available in headers.
         Try --with-boost-asio if need be.]))
 ])