Specify some compile options in _tags instead of Makefile.

src/Makefile

@@ -19,7 +19,7 @@ endif
 # ----------------------------------------------------------------------------
 # Ocamlbuild tool and settings.
 
-OCAMLBUILD := ocamlbuild -classic-display -j 0 -cflags "-safe-string -bin-annot"
+OCAMLBUILD := ocamlbuild -classic-display -j 0
 
 # ----------------------------------------------------------------------------
 # For everyday development.

src/_tags

@@ -2,6 +2,11 @@
 # This is disabled when shipping.    (my_warnings)
 <*>: my_warnings
 
+# Compile options.
+true: \
+  safe_string, \
+  bin_annot
+
 # Menhir needs the Unix library.
 <menhir.{byte,native}>: use_unix
 
@@ -21,4 +26,3 @@
 # these build directories violate its hygiene rules.
 <_build>: -traverse
 <_stage*>: -traverse
-