This commit makes e.g. make -s world.opt completely
silent when everything works, in order to increase the visibility of
any unexpected message occurring during the build.
This will be useful for instance during CI, in particular it should
make it easier to catch undefined build variables.
The implementation is straightforward. The OCAMLLEX_FLAGS variable is
defined in Makefile.common.in and then used consistently by all
lexing recipes.
In addition, in tools/Makefile, the two rules producing the lexers
from cvt_emit.mll and make_opcodes.mll have been replaced by a
pattern-rule and the useless .SUFFIXES target has been removed.
a201ade7f3