Makefile: add dependencies to $(BUILD_DIR) goal

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>

Makefile

@@ -11,6 +11,5 @@ clean:
 	rm -rf build
 
 SRC_DIR ?= .
-# XXX: cannot add true dependencies because some file names contain spaces.
-$(BUILD_DIR):
+$(BUILD_DIR): $(filter-out README.rst contributing/project.rst experimental/%,$(shell find $(SRC_DIR) -name '*.rst'))
 	sphinx-build -W $(SRC_DIR) $(BUILD_DIR)