Makefile: make the .d rule portable

The \+ syntax is not POSIX and supported only by GNU sed.
This changes both the grep and sed commands to use extended regex.

Note: sed -E option is not POSIX either, but widely supported.

Makefile

@@ -146,8 +146,9 @@ graph.svg: Makefile
 #         dependencies (ie. the dependency graph is a DAG);
 $(OBJDIR)/%.d: $(SRCDIR)/%.f90 | $(OBJDIR)
 	@printf '$(@:d=o): $< \\\n' > '$@'
-	@grep -v 'use iso_' '$<' | \
-	 sed -n 's@^[^!].*use \([^,]\+\),.*@$(OBJDIR)/\1.o \\@p' | \
+	@grep -vE '^[[:blank:]]*use.*(intrinsic|iso_)' '$<' | \
+	sed -nE 's@^[[:blank:]]*use[[:blank:]]+'\
+	'([^,[:blank:]&]+).*@$(OBJDIR)/\1.o \\@p' | \
 	 sort -u >> '$@'
 
 # Load the generated rules