dep_autopatch: Ensure ['s `!=` operator has operands

Consider the following test:
  elif [ 0 != `find ... | xargs grep -ci rebar` ]; then

find(1) may return no file at all and xargs has nothing on its stdin. In
this case, GNU xargs (from findutils) still executes the given command
and grep(1) returns 0.

However, FreeBSD's xargs for instance does not run the command at all if
there is nothing on stdin. Therefore nothing is printed on stdout and the
test becomes:
  elif [ 0 != ]; then

This triggers a warning from the shell:
  [: !=: argument expected

Prepending both operands with a literal 'x' fixes the problem.

For consistency's sake, I modified the two other tests as well, though
I didn't have any problem with them.

core/deps.mk

@@ -67,11 +67,11 @@ distclean:: distclean-deps distclean-pkg
 # in practice only Makefile is needed so far.
 define dep_autopatch
 	if [ -f $(DEPS_DIR)/$(1)/Makefile ]; then \
-		if [ 0 != `grep -c "include ../\w*\.mk" $(DEPS_DIR)/$(1)/Makefile` ]; then \
+		if [ x0 != x`grep -c "include ../\w*\.mk" $(DEPS_DIR)/$(1)/Makefile` ]; then \
 			$(call dep_autopatch2,$(1)); \
-		elif [ 0 != `grep -ci rebar $(DEPS_DIR)/$(1)/Makefile` ]; then \
+		elif [ x0 != x`grep -ci rebar $(DEPS_DIR)/$(1)/Makefile` ]; then \
 			$(call dep_autopatch2,$(1)); \
-		elif [ 0 != `find $(DEPS_DIR)/$(1)/ -type f -name \*.mk -not -name erlang.mk | xargs grep -ci rebar` ]; then \
+		elif [ x0 != x`find $(DEPS_DIR)/$(1)/ -type f -name \*.mk -not -name erlang.mk | xargs grep -ci rebar` ]; then \
 			$(call dep_autopatch2,$(1)); \
 		else \
 			if [ -f $(DEPS_DIR)/$(1)/erlang.mk ]; then \