build: actually stop configure if Boost isn't available

If Boost is not found via AX_BOOST_BASE, we don't actually stop
configuring, only a warning is emitted:
```bash
checking for boostlib >= 1.58.0 (105800)... configure: We could not detect the boost libraries (version MINIMUM_REQUIRED_BOOST or higher). If you have a staged boost library (still not installed) please specify $BOOST_ROOT in your environment and do not give a PATH to --with-boost option.  If you are sure you have boost installed, then check your version number looking in <boost/version.hpp>. See http://randspringer.de/boost for more documentation.
```

Instead we would usually fail when one of the other
AX_BOOST_* macros fails to find a library. These macros are slowly being
removed, and in any case, it makes more sense to fail earlier if Boost
is missing.

If Boost is unavailable, the failure now looks like:
```bash
checking for boostlib >= 1.58.0 (105800)... configure: We could not detect the boost libraries (version 1.58.0 or higher). If you have a staged boost library (still not installed) please specify $BOOST_ROOT in your environment and do not give a PATH to --with-boost option.  If you are sure you have boost installed, then check your version number looking in <boost/version.hpp>. See http://randspringer.de/boost for more documentation.
configure: error: Boost is not available!
```

Note that we now just pass the version into AX_BOOST_BASE, which fixes
it's display in the output (rather than MINIMUM_REQUIRED_BOOST).

Author: fanquake

configure.ac

@@ -1377,11 +1377,8 @@ fi
 
 if test x$use_boost = xyes; then
 
-dnl Minimum required Boost version
-define(MINIMUM_REQUIRED_BOOST, 1.58.0)
-
-dnl Check for Boost libs
-AX_BOOST_BASE([MINIMUM_REQUIRED_BOOST])
+dnl Check for Boost headers
+AX_BOOST_BASE([1.58.0],[],[AC_MSG_ERROR([Boost is not available!])])
 if test x$want_boost = xno; then
     AC_MSG_ERROR([[only libbitcoinconsensus can be built without boost]])
 fi