Warn about non-contiguous clauses in the Mercury system by default.

configure.ac:
    Require the installed compiler to have the recent bug fix to
    contiguity warnings, since without that bug fix, we get warnings
    for code that is actually perfectly ok.

browser/MDB_FLAGS.in:
compiler/COMP_FLAGS.in:
deep_profiler/DEEP_FLAGS.in:
library/LIB_FLAGS.in:
mdbcomp/MDBCOMP_FLAGS.in:
mfilterjavac/MFILTERJAVAC_FLAGS.in:
profiler/PROF_FLAGS.in:
slice/SLICE_FLAGS.in:
ssdb/SSDB_FLAGS.in:
    Specify both --warn-non-contiguous-clauses and
    --warn-non-contiguous-foreign-procs as the default in each directory.

library/Mercury.options:
    Disable the warnings for exception.m and io.m, which are have not
    been cleaned up yet wrt these warnings.

mfilterjavac/MFILTERJAVAC_FLAGS.in

@@ -2,6 +2,8 @@
 --no-infer-all
 --warn-known-bad-format-calls
 --warn-unknown-format-calls
+--warn-non-contiguous-clauses
+--warn-non-contiguous-foreign-procs
 --warn-unused-imports
 --warn-insts-with-functors-without-type
 --halt-at-warn