build: Don't regenerate node symlink

The node -> out/*/node symlink is getting recreated in parallel with
other targets in the makefile which require it (e.g. test-ci) and
this seems to be causing a race condition which is showing up on AIX

Fixes: https://github.com/nodejs/node/issues/9825
PR-URL: https://github.com/nodejs/node/pull/9827
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-by: Michael Dawson <michael_dawson@ca.ibm.com>

Makefile

@@ -60,13 +60,18 @@ endif
 # to check for changes.
 .PHONY: $(NODE_EXE) $(NODE_G_EXE)
 
+# The -r/-L check stops it recreating the link if it is already in place,
+# otherwise $(NODE_EXE) being a .PHONY target means it is always re-run.
+# Without the check there is a race condition between the link being deleted
+# and recreated which can break the addons build when running test-ci
+# See comments on the build-addons target for some more info
 $(NODE_EXE): config.gypi out/Makefile
 	$(MAKE) -C out BUILDTYPE=Release V=$(V)
-	ln -fs out/Release/$(NODE_EXE) $@
+	if [ ! -r $(NODE_EXE) -o ! -L $(NODE_EXE) ]; then ln -fs out/Release/$(NODE_EXE) $@; fi
 
 $(NODE_G_EXE): config.gypi out/Makefile
 	$(MAKE) -C out BUILDTYPE=Debug V=$(V)
-	ln -fs out/Debug/$(NODE_EXE) $@
+	if [ ! -r $(NODE_EXE) -o ! -L $(node_EXE) ]; then ln -fs out/Debug/$(NODE_EXE) $@; fi
 
 out/Makefile: common.gypi deps/uv/uv.gyp deps/http_parser/http_parser.gyp deps/zlib/zlib.gyp deps/v8/build/toolchain.gypi deps/v8/build/features.gypi deps/v8/tools/gyp/v8.gyp node.gyp config.gypi
 	$(PYTHON) tools/gyp_node.py -f make