Compiler error: expected a "{" before "int main"

When trying to compile my program I get the error

icpc -c main.cpp
main.cpp(4): error: expected a "{"
int main()
^

main.cpp(55): error: return value type does not match the function type
return 0;
       ^

It seems to not be recognizing my main function as the main function. Here is the main function code, Makefile, and header:

main.cpp

#include <iostream>
#include "main.h"

int main()
{
float p,c,dt;
float P[10001], C[10001];

std::cout<<"For the following inputs, the equilibrium values for population and CO2 concentration are 120 and 340 respectively. \nEnter your plant population, p, in 100m^2: ";
std::cin>>p;
std::cout<<"Enter your initial carbon concentration, c, in ppm: ";
std::cin>>c;
std::cout<<"Enter your time interval in number of hours: ";
std::cin>>dt;
dt = 0.01*dt;

co2(c,p,dt,P,C);

int idim,jdim,i1,i2,j1,j2;
idim=100;
jdim=100;
i1=0;
i2=100;
j1=0;
j2=100;
float Clo=0.0;
float Chi=5000.0;

float TR[6] = {0.5,1.,0.,0.5,0.,1.};

plotimage(C,idim,jdim,i1,i2,j1,j2,Clo,Chi,TR);
return 0;
}

Header file:

float dcdt(float, float);
float dpdt(float, float);
void co2(float, float, float, float[], float[]);
void plot(float[], float[], float);
void plotimage(float [],int, int, int, int, int, int, float, float, float [])

Makefile:

main: main.o dcdt.o dpdt.o co2.o plot.o plotmap.o
    icpc -o main main.o dcdt.o dpdt.o co2.o plot.o plotmap.o -ltrapfpe -lpgplot -lcpgplot -lX11 

main.o: main.cpp main.h
    icpc -c main.cpp

dcdt.o: dcdt.cpp main.h
    icpc -c dcdt.cpp

dpdt.o: dpdt.cpp main.h
    icpc -c dpdt.cpp

co2.o: co2.cpp main.h
    icpc -c co2.cpp

plot.o: plot.cpp main.h
    icpc -c plot.cpp

plotmap.o: plotmap.cpp main.h
    icpc -c plotmap.cpp

You are missing a semicolon after the last function prototype in your header:

void plotimage(float [],int, int, int, int, int, int, float, float, float []);
//              Here --------------------------------------------------------^

Here is why the compiler tells you that it is looking for an opening curly brace: it sees the last line of the header (which is included verbatim in your main.cpp file) and thinks that it is looking at the beginning of a function definition, not at a forward declaration. It sees the return type, the name, and the list of parameter types, so the next thing that should come after that is an opening curly brace.