Compiler.h: remove support for GCC 4.9

Supporting GCC 4.9 is becoming cumbersome as we refactor more code to
C++14.

This shouldn't be a problem for anybody, because Debian Stable
(Stretch) has version 6.3, and Ubuntu LTS (Xenial) has version 5.3.

doc/developer.xml

@@ -47,7 +47,7 @@
       <listitem>
         <para>
           the code should be C++14 compliant, and must compile with
-          <application>GCC</application> 4.9 and
+          <application>GCC</application> 5.0 and
           <application>clang</application> 3.4
         </para>
       </listitem>

doc/user.xml

@@ -87,7 +87,7 @@ cd mpd-version</programlisting>
         <listitem>
           <para>
             a C++14 compiler (e.g. <ulink
-            url="http://gcc.gnu.org/">gcc 4.9</ulink> or <ulink
+            url="http://gcc.gnu.org/">gcc 5.0</ulink> or <ulink
             url="http://clang.llvm.org/">clang 3.9</ulink>)
           </para>
         </listitem>