Merge pull request #1622 from kini/c++14-prerequisite

Mention C++14 dependency in the manual.

doc/manual/installation/prerequisites-source.xml

@@ -10,14 +10,14 @@
 
   <listitem><para>GNU Make.</para></listitem>
 
-  <listitem><para>A version of GCC or Clang that supports C++11.</para></listitem>
+  <listitem><para>A version of GCC or Clang that supports C++14.</para></listitem>
 
   <listitem><para><command>pkg-config</command> to locate
   dependencies.  If your distribution does not provide it, you can get
   it from <link
   xlink:href="http://www.freedesktop.org/wiki/Software/pkg-config"
   />.</para></listitem>
-  
+
   <listitem><para>The OpenSSL library to calculate cryptographic hashes.
   If your distribution does not provide it, you can get it from <link
   xlink:href="https://www.openssl.org"/>.</para></listitem>