diff -Naur a/CMake/GenerateExportHeader.cmake b/CMake/GenerateExportHeader.cmake
--- a/CMake/GenerateExportHeader.cmake	2015-09-01 22:41:26.000000000 +0100
+++ b/CMake/GenerateExportHeader.cmake	2021-06-11 17:35:00.539789015 +0100
@@ -166,7 +166,7 @@
     execute_process(COMMAND ${CMAKE_C_COMPILER} ARGS --version
       OUTPUT_VARIABLE _gcc_version_info
       ERROR_VARIABLE _gcc_version_info)
-    string(REGEX MATCH "[345]\\.[0-9]\\.[0-9]*"
+    string(REGEX MATCH "([3-9]|1[0-9])\\.[0-9]\\.[0-9]*"
       _gcc_version "${_gcc_version_info}")
     # gcc on mac just reports: "gcc (GCC) 3.3 20030304 ..." without the
     # patch level, handle this here:
diff -Naur a/CMake/vtkCompilerExtras.cmake b/CMake/vtkCompilerExtras.cmake
--- a/CMake/vtkCompilerExtras.cmake	2015-09-01 22:41:26.000000000 +0100
+++ b/CMake/vtkCompilerExtras.cmake	2021-06-11 17:35:00.539789015 +0100
@@ -32,7 +32,7 @@
     OUTPUT_VARIABLE _gcc_version_info
     ERROR_VARIABLE _gcc_version_info)
 
-  string (REGEX MATCH "[345]\\.[0-9]\\.[0-9]*"
+  string (REGEX MATCH "([3-9]|1[0-9])\\.[0-9]\\.[0-9]*"
     _gcc_version "${_gcc_version_info}")
   if(NOT _gcc_version)
     string (REGEX REPLACE ".*\\(GCC\\).*([34]\\.[0-9]).*" "\\1.0"