build system: require Boost 1.49.0 or newer

Earlier versions fail to build on both my development system as well
as my CentOS 7 BuildBot CI instance. Therefore I cannot properly
support that version anymore.

See #2037.

NEWS.md

@@ -35,6 +35,12 @@
 * mkvmerge: MPEG PS reader: fixed mkvmerge trying to handle an "end" code the
   same way as a "program stream map" code.
 
+## Build system changes
+
+* Boost: the minimum required version has been bumped to 1.49.0. Earlier
+  releases fail to build on my current systems and will therefore not be
+  supported anymore.
+
 
 # Version 13.0.0 "The Juggler" 2017-06-25
 

README.md

@@ -90,7 +90,7 @@ programs and libraries you absolutely need are:
 
 - [Boost](http://www.boost.org/) — Several of Boost's libraries are
   used: `format`, `RegEx`, `filesystem`, `system`, `math`,
-  `Range`, `rational`, `variant`. At least v1.46.0 is required.
+  `Range`, `rational`, `variant`. At least v1.49.0 is required.
 
 - [libxslt's xsltproc binary](http://xmlsoft.org/libxslt/) and
   [DocBook XSL stylesheets](https://sourceforge.net/projects/docbook/files/docbook-xsl/)

ac/boost.m4

@@ -1,5 +1,5 @@
 # boost's headers must be present.
-AX_BOOST_BASE([1.46.0])
+AX_BOOST_BASE([1.49.0])
 
 # boost::system must be present.
 AX_BOOST_SYSTEM()