Error 2 when running the makefile of open-source code - makefile

Question

So I'm trying to run the open-source code from this research paper, but I keep getting [make] Error 2. The makefile for the code was thankfully provided so I all I did was run the make command when I was in the folder that contained the makefile.
I'm new to all of this, I got everything from the internet, so I apologize if I'm a bit ignorant. I'm trying to learn, please be patient (:
QuantumHoneybees-MacBook-Pro:femflip2 Joe$ make
makeCompiling src/adaptive2.cpp ...
In file included from src/adaptive2.cpp:13:
./src/kernel.h:16:20: warning: unused function 'smooth_kernel' [-Wunused-function]
static FLOAT64 smooth_kernel( FLOAT64 r2, FLOAT64 h ) {
^
In file included from src/adaptive2.cpp:15:
./src/util2.h:29:13: warning: unused function 'dump' [-Wunused-function]
static void dump( const char *format, ...) {
^
./src/util2.h:45:13: warning: unused function 'run' [-Wunused-function]
static void run( const char *format, ...) {
^
./src/util2.h:54:20: warning: unused function 'format_str' [-Wunused-function]
static const char *format_str( const char *format, ...) {
^
./src/util2.h:65:13: warning: unused function 'is_nan' [-Wunused-function]
static bool is_nan(const double v) {
^
./src/util2.h:86:17: warning: unused function 'square' [-Wunused-function]
static FLOAT64 square(FLOAT64 a) {
^
./src/util2.h:120:17: warning: unused function 'getSeconds' [-Wunused-function]
static FLOAT64 getSeconds() {
^
./src/util2.h:142:15: warning: unused function 'stretchPosition' [-Wunused-function]
static vec2d stretchPosition( const svd2 &svd, vec2d p, FLOAT64 min, FLOAT64 max, bool inverse=true ) {
^
./src/util2.h:162:28: warning: unused function 'marchPoints' [-Wunused-function]
static std::vector<vec2d> marchPoints( const std::vector<vec2d> &nodes, const std::vector<FLOAT64> &levelsets, bool fill=true ) {
^
./src/util2.h:180:17: warning: unused function 'computeVolume' [-Wunused-function]
static FLOAT64 computeVolume( const std::vector<vec2d> points ) {
^
./src/util2.h:192:17: warning: unused function 'distance' [-Wunused-function]
static FLOAT64 distance( const vec2d &p0, const vec2d &p1, vec2d &p ) {
^
./src/util2.h:222:17: warning: unused function 'detDelaunay' [-Wunused-function]
static FLOAT64 detDelaunay( vec2d p0, vec2d p1, vec2d p2, vec2d p3 ) {
^
src/adaptive2.cpp:53:13: warning: unused function 'sampleVelocity' [-Wunused-function]
static bool sampleVelocity( sorter2& sorter, vec2d pos, FLOAT64 dpx, const std::vector<particle2 *> &neighbors, vec2d &vel ) {
^
13 warnings generated.
Compiling src/ann/ANN.cpp ...
Compiling src/ann/bd_fix_rad_search.cpp ...
Compiling src/ann/bd_pr_search.cpp ...
Compiling src/ann/bd_search.cpp ...
Compiling src/ann/bd_tree.cpp ...
Compiling src/ann/brute.cpp ...
Compiling src/ann/kd_dump.cpp ...
src/ann/kd_dump.cpp:44:14: warning: unused variable 'EPSILON' [-Wunused-const-variable]
const double EPSILON = 1E-5; // small number for float comparison
^
1 warning generated.
Compiling src/ann/kd_fix_rad_search.cpp ...
Compiling src/ann/kd_pr_search.cpp ...
Compiling src/ann/kd_search.cpp ...
Compiling src/ann/kd_split.cpp ...
Compiling src/ann/kd_tree.cpp ...
Compiling src/ann/kd_util.cpp ...
Compiling src/ann/perf.cpp ...
Compiling src/annsorter2.cpp ...
In file included from src/annsorter2.cpp:9:
In file included from ./src/annsorter2.h:10:
In file included from ./src/ann2.h:11:
./src/util2.h:29:13: warning: unused function 'dump' [-Wunused-function]
static void dump( const char *format, ...) {
^
./src/util2.h:45:13: warning: unused function 'run' [-Wunused-function]
static void run( const char *format, ...) {
^
./src/util2.h:54:20: warning: unused function 'format_str' [-Wunused-function]
static const char *format_str( const char *format, ...) {
^
./src/util2.h:65:13: warning: unused function 'is_nan' [-Wunused-function]
static bool is_nan(const double v) {
^
./src/util2.h:86:17: warning: unused function 'square' [-Wunused-function]
static FLOAT64 square(FLOAT64 a) {
^
./src/util2.h:120:17: warning: unused function 'getSeconds' [-Wunused-function]
static FLOAT64 getSeconds() {
^
./src/util2.h:142:15: warning: unused function 'stretchPosition' [-Wunused-function]
static vec2d stretchPosition( const svd2 &svd, vec2d p, FLOAT64 min, FLOAT64 max, bool inverse=true ) {
^
./src/util2.h:162:28: warning: unused function 'marchPoints' [-Wunused-function]
static std::vector<vec2d> marchPoints( const std::vector<vec2d> &nodes, const std::vector<FLOAT64> &levelsets, bool fill=true ) {
^
./src/util2.h:180:17: warning: unused function 'computeVolume' [-Wunused-function]
static FLOAT64 computeVolume( const std::vector<vec2d> points ) {
^
./src/util2.h:192:17: warning: unused function 'distance' [-Wunused-function]
static FLOAT64 distance( const vec2d &p0, const vec2d &p1, vec2d &p ) {
^
./src/util2.h:222:17: warning: unused function 'detDelaunay' [-Wunused-function]
static FLOAT64 detDelaunay( vec2d p0, vec2d p1, vec2d p2, vec2d p3 ) {
^
11 warnings generated.
make: *** [all] Error 2
QuantumHoneybees-MacBook-Pro:femflip2 Joe$ make
Edit:: I've narrowed it down to the makefile in lines 36-44 there is something wrong with what is written. I unfortunately have zero experience with writing makefiles, so I don't know how to fix it. I tried messing around with some lines' indentation because some other threads seem to have that issue, but to no avail.

Answer 1

There's a missing include in fluid2.h, NULL isn't defined.
Either invoke
make OPT="-Wall -Os -include cstddef"
Or add
#include <cstddef>
to the includes in fluid2.h.
It's quicker to just rewrite the terrible included makefile, try this:
CC := g++
CPPFLAGS := -Isrc -Isrc/ann -Isrc/pcgsolver -MMD -MP
CXXFLAGS := -Wall -Os
LDFLAGS := -framework OpenGL -framework GLUT
sources := $(wildcard src/*.cpp src/ann/*.cpp)
objects := $(sources:.cpp=.o)
deps := $(objects:.o=.d)
flip2d: $(objects)
$(LINK.o) $^ $(LDLIBS) -o $#
.PHONY: clean
clean: ; $(RM) $(objects) $(deps) flip2d
-include $(deps)