Boost.System has been headers only since 1.69.0 (#911)

The recently released Boost 1.89.0 removed the compatibility stub, so requiring
the BOOST_SYSTEM library will now lead to a build failure.

This commit also bumps the minimum version of Boost to 1.69.0, as that was the
first version where this is headers only.

Author: arjendekorte

configure.ac

@@ -56,8 +56,7 @@ yes)	DEBUG_FLAGS="-g -Wall -O0"
 esac
 
 
-AX_BOOST_BASE([1.60], [], [AC_MSG_ERROR([Boost libraries are required])])
-AX_BOOST_SYSTEM
+AX_BOOST_BASE([1.69], [], [AC_MSG_ERROR([Boost libraries are required])])
 AX_BOOST_THREAD
 CXXFLAGS="$CXXFLAGS $BOOST_CPPFLAGS"