Revise use of libregex in Windows branch

[jimklimov]

In m4 recipes it is detected as part of USB support in Windows-branch m4/nut_check_libusb.m4

A $(LIBREGEX_LIBS) Makefile variable (defined by configure.ac) seems to be only used explicitly in recipe to build apcsmart driver.

However a regcomp() method is also used in macosc-ups.c and usb-common.c sources.

This issue is about verifying if we need it at all, or duplicate effort with another regex implementation (we have code for USB VendorID/ProductID regex matching, etc., for example).

[jimklimov]

Note: seems to be also used by libnetsnmp builds, so maybe unavoidable (at least on Windows target).