Compiler.h: require gcc 4.7 or newer

The ScopeExit library uses C++11 initializers, which gcc 4.6 does not
support.  Let's kill support for this ancient incomplete C++11
compiler, nobody should be using it anymore.

INSTALL

@@ -12,7 +12,7 @@ install MPD.  If more information is desired, read the user manual:
 Dependencies
 ------------
 
-gcc 4.6 or later - http://gcc.gnu.org/
+gcc 4.7 or later - http://gcc.gnu.org/
 clang 3.2 or later - http://clang.llvm.org/
 Any other C++11 compliant compiler should also work.
 

NEWS

@@ -3,6 +3,7 @@ ver 0.19.18 (not yet released)
   - ffmpeg: fix crash with older FFmpeg versions (< 3.0)
   - ffmpeg: log detailed error message
   - ffmpeg: support FFmpeg 3.1
+* require gcc 4.7 or newer
 
 ver 0.19.17 (2016/07/09)
 * decoder

doc/developer.xml

@@ -41,7 +41,7 @@
       <listitem>
         <para>
           the code should be C++11 compliant, and must compile with
-          <application>GCC</application> 4.6 and
+          <application>GCC</application> 4.7 and
           <application>clang</application> 3.2
         </para>
       </listitem>

src/Compiler.h

@@ -45,7 +45,7 @@
 #    error Sorry, your clang version is too old.  You need at least version 3.1.
 #  endif
 #elif defined(__GNUC__)
-#  if GCC_OLDER_THAN(4,6)
+#  if GCC_OLDER_THAN(4,7)
 #    error Sorry, your gcc version is too old.  You need at least version 4.6.
 #  endif
 #else