https://github.com/qgis/QGIS/commit/a20c909c1b2bd0dfcae281fb6e55226d3d3dc423

relax flex requirement as QGIS builds and runs fine with our flex.

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -329,7 +329,7 @@ if(WITH_CORE)
     set (USING_NINJA TRUE)
   endif()
 
-  find_package(FLEX 2.6 REQUIRED)
+  find_package(FLEX 2.5.39 REQUIRED)
   find_package(BISON REQUIRED)
 
   #############################################################