Compilation doesn't work when including library

In order to compile a program using threads I have to use next line:
cc -lpthread -o thr_test thr_test.c

In order to compile a program containing a certain library (NTL made by Shoup), I need next line:
g++ -I../include -I. -O2 -o DTMS_TST DTMS_TST.c ntl.a -lm

When I want to combine both, ex: using threads in the program using the NTL library, I always have compilation errors like:
(using NTL in the thread programme)
ERROR:
thr_test.c:3:20: NTL/ZZ.h: No such file or directory

If I use next line (inclusion of library), I get next errors:
cc -I../include -I. -lpthread -o thr_test thr_test.c ntl.a

I get a whole list of conflicts, parse errors,...

Is there somebody that can hekp me out?

swakke