Fix check-git-abc Makefile target

Makefile

@@ -798,6 +798,10 @@ check-git-abc:
 	    echo "3. Initialize the submodule: Run 'git submodule update --init' to set up 'abc' as a submodule."; \
 	    echo "4. Reapply your changes: Move your saved changes back to the 'abc' directory, if necessary."; \
 	    exit 1; \
+	elif [ -n "$(git submodule status abc | grep '^-' )" ]; then \
+	    echo "Error: The 'abc' directory does not exist."; \
+			echo "Initialize the submodule: Run 'git submodule update --init' to set up 'abc' as a submodule."; \
+	    exit 1; \
 	else \
 	    echo "'abc' comes from a tarball. Continuing."; \
 	    exit 0; \

README.md

@@ -123,6 +123,8 @@ Makefile.
 	$ vi Makefile            # ..or..
 	$ vi Makefile.conf
 
+Initialize the `abc` submodule with `git submodule update --init`.
+
 To build Yosys simply type 'make' in this directory.
 
 	$ make