Makefile Issues with C++ & Gtkmm - c++

Question

I am familiar with Gtkmm and C++, but new to Makefiles and am having issues getting mine to actually produce an executable. I know a full Makefile is overkill for the current simplicity of my project, but I am trying to put together a basic template for use in larger projects. I am on Ubuntu 17.10 and running the make command from my "build" folder. Any help would be greatly appreciated!
Terminal Output
pearscol#UBUNTU-1710:~/projects/testprogram/build$ make
g++ -o testprogram `pkg-config --libs gtkmm-3.0`
/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o: In
function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
Makefile:19: recipe for target 'testprogram' failed
make: *** [testprogram] Error 1
Source File Structure
- /build
- /build/Makefile
- /src
- /src/main.cpp
/build/Makefile
CXX = g++
CXXFLAGS = `pkg-config gtkmm-3.0 --cflags` -std=c++14 -g -Wall
INCLUDES = -I../include
SRCS = $(wildcard *.cpp)
OBJS = $(SRCS:.cpp=.o)
PROGRAM = testprogram
DEPDIR = ../src/
LIBS = `pkg-config --libs gtkmm-3.0`
all: $(PROGRAM)
-include $(OBJS:%.o=$(DEPDIR)/%.Po)
%.o: %.cpp
$(CXX) -MT $# -MD -MP -MF $*.Tpo -c -o $# $(CXXFLAGS) $<
$(PROGRAM): $(OBJS)
$(CXX) -o $(PROGRAM) $(OBJS) $(LIBS)
# DO NOT DELETE THIS LINE -- make depend needs it
/src/main.cpp
#include <gtkmm.h>
int main(int argc, char *argv[]) {
Glib::RefPtr<Gtk::Application> app =
Gtk::Application::create(argc, argv, "testprogram");
Gtk::Window window;
window.set_default_size(400, 200);
window.set_title("Tutorial");
return app->run(window);
}

Answer 1

If you want to specify the source files using a $(wildcard ...) function,
then replace:
SRCS = $(wildcard *.cpp)
with:
SRCS = $(wildcard src/*.cpp)
because src is where the *.cpp files are, with respect to the build directory.
To place the object files in the build directory, then replace:
OBJS = $(SRCS:.cpp=.o)
with:
$(notdir $(SRCS:.cpp=.o))
Finally do one of the following:
Either replace the pattern rule:
%.o: %.cpp
with:
%.o: src/%.cpp
Or, leave the pattern rule as it is and add:
VPATH := src
to the variable settings at the top of your makefile. See
VPATH.
Asides
You default target all is a phony target: you are never going to make
a file called all, and you assume no such file will ever exist. Better specify it as a .PHONY target.
For defining variables, use simple expansion - VAR := val - rather than
recursive expansion - VAR = val - unless recursive expansion is needed, which it isn't in your makefile.
Your makefile lacks a clean target for
deleting all make-prodicts and restoring the build directory to its pristine
state. It is standard to provide this phony target.

Answer 2

As far as I can tell,
$(CXX) -o $(PROGRAM) $(OBJS) $(LIBS)
results in
g++ -o testprogram `pkg-config --libs gtkmm-3.0`
because $(OBJS) is empty, because $(SRCS) is empty, because *.cpp matches no files, because your build directory contains no .cpp files.