misc/mbuffer: Fix build if www/libwww installed

Remove libmd5 from AC_SEARCH_LIBS and only searches for libmd,
avoid linking with libmd5 installed by www/libwww.

PR: 273735
Reported by: meta
MFH: 2023Q3 (build fix)

(cherry picked from commit d3f22d319918c1dda4dbfbc5d85678c91ed13590)