[LyX/master] Bump minimal GCC version to 4.9

[Yuriy Skalko]

commit 9b55d2f71e34e2ea73efb7ceb017a39b9623f543
Author: Yuriy Skalko <yuriy.skalko at gmail.com>
Date:   Mon Nov 16 19:35:54 2020 +0200

    Bump minimal GCC version to 4.9
---
 INSTALL                 |    2 +-
 README                  |    2 +-
 lib/doc/Development.lyx |   10 ++++++++++
 3 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/INSTALL b/INSTALL
index 2861d7b..6f26ee2 100644
--- a/INSTALL
+++ b/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++
diff --git a/README b/README
index 222a993..a0a44dc 100644
--- a/README
+++ b/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
diff --git a/lib/doc/Development.lyx b/lib/doc/Development.lyx
index 4e588ac..1ce4be3 100644
--- a/lib/doc/Development.lyx
+++ b/lib/doc/Development.lyx
@@ -5363,6 +5363,16 @@ 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
  you have better ideas, discuss them on the developer's list before writing