Make 'build' target phony once again

In commit 5fb4b21 ("Refine deploy target..."), the 'build' target was
made normal, i.e. non-phony, but on further review it does in fact make
sense to declare 'build' phony, such that it is run no matter the status
of the root-level 'build' directory, but for different reasons.

Previously, we had been considering the presence of 'build' directory as
a reasonable proxy for determining whether the `./gradlew build` had
been run. If the directory was present, we considered the 'build' target
up-to-date. If not, then we would re-run `./gradlew build`. This is all
sensible enough, except for the fact that the root-level 'build'
directory has almost nothing to do with the actual output of `./gradlew
build`. Gradle does output 'build' directories, but in the respective
subdirectory for each module of the project. After `./gradlew build` has
been run, we would see a 'desktop/build' directory, a 'seednode/build'
directory and so forth. It just so happens that a root-level 'build'
directory was getting created at all due to idiosyncracies of a
particular Kotlin plugin.

This commit updates the makefile to better respect this reality by:

 - preserving the 'build' target but marking it once again as PHONY

 - introducing new 'seednode/build' and 'desktop/build' targets that
   trigger './gradlew :seednode:build` and ./gradlew :desktop:build`
   commands respectively.

 - making 'build' depend on these two new targets

In light of this realization of flawed thinking about the root-level
build dir, this change also restores `make clean` to calling `./gradlew
clean` instead of `rm -rf build`.

Makefile

@@ -108,15 +108,19 @@ setup: build .localnet
 clean: clean-build clean-localnet
 
 clean-build:
-	rm -rf build
+	./gradlew clean
 
 clean-localnet:
 	rm -rf .localnet ./dao-setup
 
-# Build all Bisq binaries and generate the shell scripts used to run
-# them in the targets below
-build:
-	./gradlew build
+# Build Bisq binaries and shell scripts used in the targets below
+build: seednode/build desktop/build
+
+seednode/build:
+	./gradlew :seednode:build
+
+desktop/build:
+	./gradlew :desktop:build
 
 # Unpack and customize a Bitcoin regtest node and Alice and Bob Bisq
 # nodes that have been preconfigured with a blockchain containing the
@@ -171,7 +175,7 @@ bitcoind: .localnet
 		-datadir=.localnet/bitcoind \
 		-blocknotify='.localnet/bitcoind/blocknotify %s'
 
-seednode: build
+seednode: seednode/build
 	./bisq-seednode \
 		--baseCurrencyNetwork=BTC_REGTEST \
 		--useLocalhostForP2P=true \
@@ -184,7 +188,7 @@ seednode: build
 		--userDataDir=.localnet \
 		--appName=seednode
 
-seednode2: build
+seednode2: seednode/build
 	./bisq-seednode \
 		--baseCurrencyNetwork=BTC_REGTEST \
 		--useLocalhostForP2P=true \
@@ -197,7 +201,7 @@ seednode2: build
 		--userDataDir=.localnet \
 		--appName=seednode2
 
-mediator: build
+mediator: desktop/build
 	./bisq-desktop \
 		--baseCurrencyNetwork=BTC_REGTEST \
 		--useLocalhostForP2P=true \
@@ -244,4 +248,4 @@ block:
 				-rpcpassword=bsq \
 				generatetoaddress 1
 
-.PHONY: seednode
+.PHONY: build seednode