busybox: add glibc dependency for vi regex option

busybox: add glibc dependency for vi regex option

Build with musl libc fails with BUSYBOX_DEFAULT_FEATURE_VI_REGEX_SEARCH
enabled. Enabling BusyBox's vi regex search option depends upon GNU
regex.  Musl libc does not support GNU regex[1].

So this patch adds explicit dependency on GNU libc and while at it
remove the FIXME comment.

1. https://wiki.musl-libc.org/functional-differences-from-glibc.html

Ref: https://dev.archive.openwrt.org/ticket/21741.html
Ref: https://forum.openwrt.org/t/busybox-not-compiling/
Ref: https://github.com/openwrt/packages/issues/4453
Signed-off-by: Satadru Pramanik <satadru@umich.edu>
[commit subject/description tweaks, From: fix, USE_GLIBC fix, removed comments]
Signed-off-by: Petr Štetiar <ynezz@true.cz>