Build: exclude regex when already provided by the system

[Coeur]

Description

nzbget includes a quite old version of the glibc regex.
Building and using such old version of regex is unnecessary in most platforms, as the latest and better versions of regex are included in all POSIX distributions for decades.

The old embedded regex is showing various concerning issues:

• potential memory leaks in re_dfa_add_node from the re_realloc calls
• dereference of null pointer in create_initial_state
• dereference of null pointer in calc_first
• dereference of null pointer in duplicate_tree
• potential memory leaks in re_copy_regs from the re_realloc calls
• dereference of null pointer in re_search_internal
• attempt to free released memory in prune_impossible_nodes
• assigned value is garbage or undefined in build_trtable
• use of function alloca is discouraged; there is no way to check for failure but failure may still occur, resulting in a possibly exploitable security vulnerability

Hence the best initial action is to avoid using this old code altogether when it's not necessary.

[dnzbk]

Thank you!