Bump minimal GCC version to 4.9

INSTALL

@@ -53,7 +53,7 @@ Requirements
 ------------
 
 First of all, you will need a C++11 standard conforming compiler, like gcc (at
-least 4.8) or clang.
+least 4.9) or clang.
 
 LyX makes great use of the C++ Standard Template Library (STL).
 This means that gcc users will have to install the relevant libstdc++

README

@@ -88,7 +88,7 @@ What do I need to compile LyX from the source distribution?
 
     * A C++11 compiler. Development is being done mainly with gcc/g++,
       but clang and MSVC are known to work too. As of LyX 2.4.0, you
-      need at least gcc 4.7.
+      need at least gcc 4.9.
 
     * The Qt library, at least version 5.2 (5.6 recommended). It is
       still possible to compile with Qt 4.8, but this is not

lib/doc/Development.lyx

@@ -5362,6 +5362,16 @@ noprefix "false"
 .
 \end_layout
 
+\begin_layout Itemize
+As of LyX 2.4.0, you can use features of C++11.
+ Accordingly you have to use C++11 standard conforming compiler, e.
+\begin_inset space \thinspace{}
+\end_inset
+
+g.
+ not too dated version of GCC or Clang.
+\end_layout
+
 \begin_layout Itemize
 Adapt the code to the structures already existing in \SpecialChar LyX
 , or in the case that