Makefile: Add .install.* to .PHONY

Also consolidate the install.tools entry in the .PHONY block at the
end of the Makefile.  The $(var:pattern=replacement) syntax is
documented in [1].

[1]: https://www.gnu.org/software/make/manual/html_node/Text-Functions.html#index-patsubst-1

Signed-off-by: W. Trevor King <wking@tremily.us>

Makefile

@@ -40,6 +40,8 @@ DOC_FILENAME	?= oci-image-spec
 
 EPOCH_TEST_COMMIT ?= v0.2.0
 
+TOOLS := esc gitvalidation glide glide-vc
+
 default: check-license lint test
 
 help:
@@ -109,9 +111,7 @@ else
 	git-validation -v -run DCO,short-subject,dangling-whitespace -range $(EPOCH_TEST_COMMIT)..HEAD
 endif
 
-.PHONY: install.tools
-
-install.tools: .install.esc .install.gitvalidation .install.glide .install.glide-vc
+install.tools: $(TOOLS:%=.install.%)
 
 .install.esc:
 	go get -u github.com/mjibson/esc
@@ -129,6 +129,8 @@ clean:
 	rm -rf *~ $(OUTPUT_DIRNAME)
 
 .PHONY: \
+	install.tools \
+	$(TOOLS:%=.install.%) \
 	validate-examples \
 	check-license \
 	clean \