Sources from subdirectories in Makefile

This should do it:

SOURCES = $(wildcard *.cpp) $(wildcard */*.cpp)

If you change you mind and want a recursive solution (i.e. to any depth), it can be done but it involves some of the more powerful Make functions. You know, the ones that allow you to do things you really shouldn't.

EDIT:
Jack Kelly points out that $(wildcard **/*.cpp) works to any depth, at least on some platforms, using GNUMake 3.81. (How he figured that out, I have no idea.)

Recursive wildcards can be done purely in Make, without calling the shell or the find command. Doing the search using only Make means that this solution works on Windows as well, not just *nix.

# Make does not offer a recursive wildcard function, so here's one:
rwildcard=$(wildcard $1$2) $(foreach d,$(wildcard $1*),$(call rwildcard,$d/,$2))

# How to recursively find all files with the same name in a given folder
ALL_INDEX_HTMLS := $(call rwildcard,foo/,index.html)

# How to recursively find all files that match a pattern
ALL_HTMLS := $(call rwildcard,foo/,*.html)

The trailing slash in the folder name is required. This rwildcard function does not support multiple wildcards the way that Make's built-in wildcard function does, but adding that support would be straightforward with a couple more uses of foreach.