compiling C project with makefile - c

Question

I have a directory called reliability:
ls reliability
analyze.c appl.sh constr.c creer.c greedys.c Makefile
Now I want to install and compile it:
First of all the content of Makefile is:
CC = gcc
CFLAGS = -g -Wall -pthread
SRCS = constr.c creer.c analyze.c greedys.c
PROG = constr creer analyze greedys
all: $(PROG)
constr: constr.c
$(CC) $(CFLAGS) -o constr constr.c
creer: creer.c
$(CC) $(CFLAGS) -o creer creer.c
analyze: analyze.c
$(CC) $(CFLAGS) -o analyze analyze.c
greedys: greedys.c
$(CC) $(CFLAGS) -o greedys greedys.c
clean:
rm -f $(PROG)
When I start compiling it:
make -f Makefile
The error occurs:
gcc -g -Wall -pthread -o constr constr.c
/tmp/cca4NKQl.o: In function `main':
/home/t1/Desktop/reliability/constr.c:89: undefined reference to `exp'
/home/t1/Desktop/reliability/constr.c:89: undefined reference to `exp'
/home/t1/Desktop/reliability/constr.c:89: undefined reference to `pow'
/home/t1/Desktop/reliability/constr.c:89: undefined reference to `log'
collect2: ld returned 1 exit status
make: *** [constr] Error 1
I pass these errors by adding -lm to gcc,
Even when I change CFLAGS from Makefile to CFLAGS = -g -Wall -pthread -lm there is also that error.
My question:
Should I compile all .c programs separately now?
Thank you I really need help,
UPDATE
When I compile creer.c there is an error:
creer.c:39:10: warning: unused variable ‘val2’ [-Wunused-variable] creer.c:38:7:
warning: unused variable ‘val’ [-Wunused-variable]
Part of creer.c is:
void values(){
int val;
double val2;
FILE *fp;
fp = fopen("instances","r");
fseek(fp,0,SEEK_SET);
if(fscanf(fp,"%d",&p)==EOF){printf("EOF\n");}
if(fscanf(fp,"%d",&m)==EOF){printf("EOF\n");}
if(fscanf(fp,"%d",&K)==EOF){printf("EOF\n");}
if(fscanf(fp,"%lf",&lambda_com)==EOF){printf("EOF\n");}
if(fscanf(fp,"%d",&num_inst)==EOF){printf("EOF\n");}
if(fscanf(fp,"%d",&boundl)==EOF){printf("EOF\n");}
if(fscanf(fp,"%d",&boundp)==EOF){printf("EOF\n");}
fclose(fp);
}

Answer 1

You need to add that linker option -lm to the end of your command line.
Do something like this:
LDFLAGS=-lm
...
constr: constr.c
$(CC) $(CFLAGS) -o constr constr.c $(LDFLAGS)
Or you could define a pattern rule:
%: %.c
$(CC) $(CFLAGS) -o $# $< $(LDFLAGS)
and use it to replace all those rules for constr, creer, and so on.