It's not clear to anyone why this variable existed in the first place,
so it was decided that it should be removed.
Most of the time, AC_CHECK_LIB and AC_SEARCH_LIBS will already alter
LIBS to prepend to it the found library, so there is no real reason to
duplicate this mechanism.
Some manual assignments have been kept because either the
action-if-found was set to no-op, or because new found libraries were
appended to MUTTLIBS, so this keeps the diff minimal with the same
"behaviour".