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>
(cherry picked from commit f1410902e6c089a53935ab44e83207e877445bc5)

package/utils/busybox/config/editors/Config.in

@@ -126,8 +126,9 @@ config BUSYBOX_CONFIG_FEATURE_VI_SEARCH
 
 config BUSYBOX_CONFIG_FEATURE_VI_REGEX_SEARCH
 	bool "Enable regex in search and replace"
-	default BUSYBOX_DEFAULT_FEATURE_VI_REGEX_SEARCH   # Uses GNU regex, which may be unavailable. FIXME
+	default BUSYBOX_DEFAULT_FEATURE_VI_REGEX_SEARCH
 	depends on BUSYBOX_CONFIG_FEATURE_VI_SEARCH
+	depends on USE_GLIBC
 	help
 	Use extended regex search.