Makefile issue: smart way to scan directory tree for .c files

I am doing a project which is growing pretty fast and keeping the object files up date is no option. The problem beyond wildcard command lies somewhere between "I do not want recursive makefiles" and "I do not want it to list by hand". The objects are supposed to go into a separate directory, which works already. Note: I am not that used to makefiles, I know the basics, but everything beyond...

So my question: How to scan a src folder recursively and do that in a smart manner?

I already did it with multiple SRC variables but that's ugly and clutters the whole makefile with an increasing number of directories.

What I currently use is:

OS = Linux

VERSION = 0.0.1
CC      = /usr/bin/gcc
CFLAGS  = -Wall -g -D_REENTRANT -DVERSION=\"$(VERSION)\"
LDFLAGS = -lm `pkg-config --cflags gtk+-2.0` `pkg-config --libs gtk+-2.0`

BUILDDIR = build
SOURCEDIR = src
HEADERDIR = src

SOURCES = $(wildcard $(SOURCEDIR)/*.c)
OBJECTS = $(patsubst $(SOURCEDIR)/%.c, $(BUILDDIR)/%.o, $(SOURCES))

NAME = cinnamon
BINARY = cinnamon.bin

ECHO = echo
RM = rm -rf
MKDIR = mkdir
INSTALL = install

.PHONY: all clean setup

all: $(BINARY)


$(BINARY): $(BUILDDIR)/$(OBJECTS)
    $(CC) $(CFLAGS) $(LDFLAGS) -I$(HEADERDIR) -I$(SOURCEDIR) $(OBJECTS) -o $(BINARY) 


$(BUILDDIR)/%.o: $(SOURCEDIR)/%.c
    $(CC) $(CFLAGS) $(LDFLAGS) -I$(HEADERDIR) -I$(SOURCEDIR) -c $< -o $@

setup:
    $(MKDIR) -p $(BUILDDIR)

install:
    $(INSTALL) -m 755 -o 0 -g 0 -d $(DESTDIR)/usr/local/bin/
    $(INSTALL) -m 755 -o 0 -g 0 $(BINARY) $(DESTDIR)/usr/local/bin/$(BINARY)
    $(INSTALL) -m 755 -o 0 -g 0 -d $(DESTDIR)/usr/local/$(NAME)/ui/
    $(INSTALL) -m 644 -o 0 -g 0 ./ui/*.ui $(DESTDIR)/usr/local/$(NAME)/ui/
#   $(INSTALL) -m 755 -o 0 -g 0 -d $(DESTDIR)/usr/local/$(NAME)/model/
#   $(INSTALL) -m 644 -o 0 -g 0 ./model/*.model $(DESTDIR)/usr/local/$(NAME)/model/

clean:
    $(RM) $(BINARY) $(OBJECTS)

distclean: clean


help:
    @$(ECHO) "Targets:"
    @$(ECHO) "all     - buildcompile what is necessary"
    @$(ECHO) "clean   - cleanup old .o and .bin"
    @$(ECHO) "install - not yet fully supported"

Thanks to answer #1 it boils down to how to solve this:

$(BUILDDIR)/%.o: $(SOURCEDIR)/%.c
    $(CC) $(CFLAGS) $(LDFLAGS) $(SOURCETREE) -c $< -o $@

especially in the case of BUILDDIR = build and SOURCEDIR having to be replaced with the single .c files from SOURCES including their paths :/