Document gcc 5 dependency.

https://www.mail-archive.com/lyx-devel@lists.lyx.org/msg223072.html

INSTALL

@@ -53,7 +53,7 @@ Requirements
 ------------
 
 First of all, you will need a C++11 standard conforming compiler, like g++ (at
-least 4.9, to have proper srd::regex) or clang++.
+least 5.0, to have proper std::regex & std::string) or clang++.
 
 LyX makes great use of the C++ Standard Library. This means that gcc
 users will have to install the relevant libstdc++ library to be able

README

@@ -87,8 +87,8 @@ How do I upgrade from an earlier LyX version?
 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 4.9.
+      but clang and MSVC are known to work too. As of LyX 2.5.0, you
+      need at least gcc 5.0.
 
     * The Qt library, at least version 5.2 (5.6 recommended). It is
       also possible to compile with Qt 6.x.

lib/RELEASE-NOTES

@@ -38,6 +38,8 @@
 
 - The support for (long unmaintained) eLyXer was removed.
 
+- When compiling with GCC, LyX know needs at least version >= 5.
+
 
 !!Known issues in version 2.5.0