Fix the target all of the Makefile (#914)

The PR #887 was wrong... The target @check of `Dune` doesn't build
all the binaries, it builds only binary artefacts `cmi`, `cmx`, ... but
no linkage is performed.

This commit mostly reverts the changes. As the sources are located in
`src/`, I use the target `@src/all`. Now the command `make` will build
everything in the directory `src`. I also add `examples` since we want
to check the libusage module still compile.

Makefile

@@ -94,7 +94,8 @@ plugins:
 # Hopefully more efficient than making "all" depend
 # on "lib" and "bin", since dune can parralelize more
 all:
-	$(DUNE) build $(DUNE_FLAGS) @check
+	$(DUNE) build $(DUNE_FLAGS) @$(SRC_DIR)/all @examples/all
+	ln -sf src/bin/text/Main_text.exe alt-ergo
 
 # declare these targets as phony to avoid name clashes with existing directories,
 # particularly the "plugins" target