make: Nothing to be done for 'all'

Question

I have only 2 files in my folder - Gnuplot_example.cpp and Makefile.

Gnuplot_example.cpp

#include "Snap.h"
int main(int argc, char* argv[]) {
TIntH expCount, PoissonCount;
TRnd a;
double binSize=0.1;
//exponential distribution  
for (int i=0; i<10000; ++i)
{
    double num=a.GetExpDev(1);
    expCount.AddDat((int)(num/binSize))++;
}

//Poisson distribution  
for (int i=0; i<10000; ++i)
{
    double num=a.GetPoissonDev(10);
    PoissonCount.AddDat((int)(num/binSize))++;
}


expCount.Sort(true,true);
PoissonCount.Sort(true,true);

{
    TVec<TPair<TFlt, TFlt > > XY1, XY2;
    for (int i=0; i<expCount.Len(); ++i)
        XY1.Add(TFltPr(expCount.GetKey(i)*binSize, expCount[i]+0.0));
    for (int i=0; i<PoissonCount.Len(); ++i)
           XY2.Add(TFltPr(PoissonCount.GetKey(i)*binSize,PoissonCount[i]+0.0));
    TGnuPlot Gp("distribution", "Exponential and Poisson Distribution");
    Gp.AddPlot(XY1, gpwLinesPoints, "Exponential");
    Gp.AddPlot(XY2, gpwLinesPoints, "Poisson");
    Gp.SetXYLabel("value", "count");
    Gp.SavePng(); //or Gp.SaveEps();
}

return 0;
}

Makefile

#
# Makefile for non-Microsoft compilers
#

all: $(Gnuplot_example)

# COMPILE
$(Gnuplot_example): $(Gnuplot_example).cpp $(DEPH) $(DEPCPP) $(EXSNAP)/Snap.o 
$(CC) $(CXXFLAGS) -o $(Gnuplot_example) $(Gnuplot_example).cpp $(DEPCPP) $(EXSNAP)/Snap.o -I$(EXSNAP) -I$(EXSNAPADV) -I$(EXGLIB) -I$(EXSNAPEXP) $(LDFLAGS) $(LIBS)

$(EXSNAP)/Snap.o: 
make -C $(EXSNAP)

clean:
rm -f *.o  $(Gnuplot_example)  $(Gnuplot_example).exe
rm -rf Debug Release

When I am using command 'make' in this folder, it says: make: Nothing to be done for 'all'. and there is no object file created. I have gone through other answers and I made sure to use tab in rules instead of spaces. I am new to this. I am not getting what is wrong. Thanks for any Help!!