Make configure detect old Boost.

This minimum version has for some time been documented as 1.55, but
its always easier to make the configure command error out if you use
an older version and while 1.55 is from 2013, it still happens.

So lets make sure we fail configure on a too old boost version.

configure.ac

@@ -592,7 +592,7 @@ fi
 if test x$use_boost = xyes; then
 
 dnl Minimum required Boost version
-define(MINIMUM_REQUIRED_BOOST, 1.47.0)
+define(MINIMUM_REQUIRED_BOOST, 1.55.0)
 
 dnl Check for boost libs
 AX_BOOST_BASE([MINIMUM_REQUIRED_BOOST])