Makefile: rm $(SOURCES), mark targets as PHONY

Since go has its own way to track dependencies and rebuild if needed,
and it is efficient enough, let's drop using SOURCES variable, mark
all targets as PHONY and let golang do its job.

The primary motivation for this was concern about using find on every
make invocation to build the list of all sources.

Some unscientific performance analisys:

Before:
> $ time make
> make: 'runc' is up to date.
>
> real	0m0.202s
> user	0m0.177s
> sys	0m0.031s

After:
> $ time make
> go build -mod=vendor -buildmode=pie  -tags "seccomp selinux apparmor" -ldflags "-X main.gitCommit="5a8210a58bd0f07cc987e6201b4174e5b93fa115" -X main.version=1.0.0-rc10+dev " -o runc .
>
> real	0m0.149s
> user	0m0.315s
> sys	0m0.106s

So, it is slightly faster using the wall clock, uses more CPU, but
we can be sure the binary is always up to date.

This also fixes the Makefile to mark all targets as PHONY. The list
was generated by `grep -E '^[a-z-]+:' Makefile | sed 's/:.*//'`.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Makefile

@@ -1,12 +1,6 @@
-.PHONY: all shell dbuild man release \
-	    localtest localunittest localintegration \
-	    test unittest integration \
-	    cross localcross vendor verify-dependencies
-
 CONTAINER_ENGINE := docker
 GO := go
 
-SOURCES := $(shell find . 2>&1 | grep -E '.*\.(c|h|go)$$')
 PREFIX := $(DESTDIR)/usr/local
 BINDIR := $(PREFIX)/sbin
 GIT_BRANCH := $(shell git rev-parse --abbrev-ref HEAD 2>/dev/null)
@@ -35,17 +29,15 @@ SHELL := $(shell command -v bash 2>/dev/null)
 
 .DEFAULT: runc
 
-runc: $(SOURCES)
+runc:
 	$(GO_BUILD) -o runc .
 
 all: runc recvtty
 
-recvtty: contrib/cmd/recvtty/recvtty
-
-contrib/cmd/recvtty/recvtty: $(SOURCES)
+recvtty:
 	$(GO_BUILD) -o contrib/cmd/recvtty/recvtty ./contrib/cmd/recvtty
 
-static: $(SOURCES)
+static:
 	$(GO_BUILD_STATIC) -o runc .
 	$(GO_BUILD_STATIC) -o contrib/cmd/recvtty/recvtty ./contrib/cmd/recvtty
 
@@ -164,3 +156,9 @@ localcross:
 	CGO_ENABLED=1 GOARCH=arm GOARM=7 CC=arm-linux-gnueabihf-gcc $(GO_BUILD) -o runc-armhf .
 	CGO_ENABLED=1 GOARCH=arm64 CC=aarch64-linux-gnu-gcc         $(GO_BUILD) -o runc-arm64 .
 	CGO_ENABLED=1 GOARCH=ppc64le CC=powerpc64le-linux-gnu-gcc   $(GO_BUILD) -o runc-ppc64le .
+
+.PHONY: runc all recvtty static release dbuild lint man runcimage \
+	test localtest unittest localunittest integration localintegration \
+	rootlessintegration localrootlessintegration shell install install-bash \
+	install-man uninstall uninstall-bash uninstall-man clean validate ci \
+	vendor verify-dependencies cross localcross