build: add stub makefiles for easier subdir builds

src/qt/Makefile

@@ -0,0 +1,9 @@
+.PHONY: FORCE
+all: FORCE
+	$(MAKE) -C .. bitcoin_qt test_bitcoin_qt
+clean: FORCE
+	$(MAKE) -C .. bitcoin_qt_clean test_bitcoin_qt_clean
+check: FORCE
+	$(MAKE) -C .. test_bitcoin_qt_check
+bitcoin-qt bitcoin-qt.exe: FORCE
+	 $(MAKE) -C .. bitcoin_qt

src/qt/test/Makefile

@@ -0,0 +1,6 @@
+all:
+	$(MAKE) -C ../../ test_bitcoin_qt
+clean:
+	$(MAKE) -C ../../ test_bitcoin_qt_clean
+check:
+	$(MAKE) -C ../../ test_bitcoin_qt_check

src/test/Makefile

@@ -0,0 +1,6 @@
+all:
+	$(MAKE) -C .. bitcoin_test
+clean:
+	$(MAKE) -C .. bitcoin_test_clean
+check:
+	$(MAKE) -C .. bitcoin_test_check