Test_pow.cpp error can't find gtest.h

Hi All:
I wanted to test pow by using this piece of test code but when I run it, I get the following error:

~/zcash-master/src/gtest > g++ -o a.out test_pow.cpp
test_pow.cpp:1:25: fatal error: gtest/gtest.h: No such file or directory
#include <gtest/gtest.h>

I looked and there is no file called gtest.h
Can anyone tell me where to get this file or explain why I can’t compile the test program for pow?