cmake: Fix flex version check for overriding the register keyword

In doxygen#11113 a check was added to override the "register" keyword for
older versions of flex. The check was added before the find_package()
call which resulted in the workaround being active for any version
of flex. This breaks the build with recent mingw-w64 which uses "register"
in its headers.

Fix by moving the version check after the find_package() call.

Author: lazka

CMakeLists.txt

@@ -107,13 +107,6 @@ endif()
 # produce compile_commands.json
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
-if (FLEX_VERSION VERSION_LESS 2.6.0)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Dregister=")
-    if (MSVC)
-        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_ALLOW_KEYWORD_MACROS=")
-    endif()
-endif()
-
 if (CMAKE_SYSTEM MATCHES "Darwin")
     set(CMAKE_OSX_DEPLOYMENT_TARGET "${MACOS_VERSION_MIN}" CACHE STRING "Minimum OS X deployment version" FORCE)
     set(CMAKE_CXX_FLAGS "-Wno-deprecated-register -mmacosx-version-min=${MACOS_VERSION_MIN} ${CMAKE_CXX_FLAGS}")
@@ -208,6 +201,12 @@ find_package(FLEX REQUIRED)
 if (FLEX_VERSION VERSION_LESS 2.5.37)
   message(SEND_ERROR "Doxygen requires at least flex version 2.5.37 (installed: ${FLEX_VERSION})")
 endif()
+if (FLEX_VERSION VERSION_LESS 2.6.0)
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Dregister=")
+    if (MSVC)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_ALLOW_KEYWORD_MACROS=")
+    endif()
+endif()
 find_package(BISON REQUIRED)
 if (BISON_VERSION VERSION_LESS 2.7)
   message(SEND_ERROR "Doxygen requires at least bison version 2.7 (installed: ${BISON_VERSION})")