CMake: make readline support configurable, not automatic

Otherwise if it's detected on the system (e.g. if you install GnuWin)
then it would always be enabled, but linking against GnuWin headers
breaks the build.

infrastructure/cmake/CMakeLists.txt

@@ -419,9 +419,13 @@ else()
 	endif()
 endif()
 
+# Enable the readline option (BOX_SUPPORT_READLINE), on by default, if readline is detected.
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}")
 find_package(Readline)
 if(READLINE_FOUND)
+	option(BOX_SUPPORT_READLINE "Enable support for linking with libreadline." ON)
+endif()
+if(BOX_SUPPORT_READLINE)
 	include_directories(${Readline_INCLUDE_DIR})
 	target_link_libraries(lib_common PUBLIC ${Readline_LIBRARY})
 endif()