Relax the requirment for the latest flex version.
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -174,7 +174,7 @@ if (APPLE)
 	use_homebrew()
 endif()
 
-find_package(FLEX 2.6)
+find_package(FLEX 2.5.39)
 set_package_properties(FLEX PROPERTIES
 	URL "https://github.com/westes/flex"
 	DESCRIPTION "The Fast Lexical Analyzer"
