[LyX/master] Update CMake for minimum GCC version: 4.9

[Yuriy Skalko]

commit b4b2d01ea7f6d63bc052f0c82c18bf6e0e08503e
Author: Yuriy Skalko <yuriy.skalko at gmail.com>
Date:   Wed Nov 25 00:00:28 2020 +0200

    Update CMake for minimum GCC version: 4.9
---
 CMakeLists.txt |   19 ++++++-------------
 1 files changed, 6 insertions(+), 13 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 808a0cc..b35f2f5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -77,9 +77,9 @@ if(NOT help AND NOT HELP)
   # 'project' triggers the searching for a compiler
   project(${LYX_PROJECT})
   if (CMAKE_COMPILER_IS_GNUCXX)
-    if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS "4.0")
+    if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS "4.9")
       message(STATUS "Gnu CXX compiler version = ${CMAKE_CXX_COMPILER_VERSION}")
-      message(STATUS "is too old, should be >= 4.0")
+      message(STATUS "is too old, should be >= 4.9")
       message(FATAL_ERROR "Exiting")
     endif()
   endif()
@@ -396,25 +396,18 @@ if(UNIX OR MINGW)
 	    #message(STATUS "dumpversion: error = ${_error}, result = ${_err}")
 	  endif()
 	  message(STATUS "Using GCC version ${GCC_VERSION}")
-	  if(GCC_VERSION VERSION_LESS 4.6)
-		message(FATAL_ERROR "gcc >= 4.6 is required.")
-	  elseif(GCC_VERSION VERSION_LESS 4.9)
-		# <regex> in gcc is unusable in versions less than 4.9.0
-		# see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53631
-		set(LYX_USE_STD_REGEX 0)
-	  else()
-		set(LYX_USE_STD_REGEX 1)
+	  if(GCC_VERSION VERSION_LESS 4.9)
+		message(FATAL_ERROR "gcc >= 4.9 is required.")
 	  endif()
+	  set(LYX_USE_STD_REGEX 1)
 	endif()
 	set(LYX_GCC11_MODE "${CXX11_FLAG}")
 else()
 	if(MSVC_VERSION LESS 1900)
 		# Drop support for msvc versions prior to 1900 (Visual Studio 2015)
 		message(FATAL_ERROR "Visual Studio >= 2015 is required.")
-		set(LYX_USE_STD_REGEX 0)
-	else()
-		set(LYX_USE_STD_REGEX 1)
 	endif()
+	set(LYX_USE_STD_REGEX 1)
 endif()
 
 if(LYX_3RDPARTY_BUILD)