gtest/gtest.h: No such file or directory #include "gtest/gtest.h"

I have never encountered a problem with testing my code using gtest a few months ago until recently I encountered a problem where all of a sudden the "gtest/gtest.h" header file can no longer be found. I am using putty to hold all my files and directories and the googletest directory is located in the same filepath as the unit test I am compiling to generate the executable. The gtest GitHub was recently updated so I'm not sure if that might have caused it. I have used git submodule add <gtest URL> and the it says that googletest already exists. I have always built my files using cmake3 ., but now I keep getting errors regarding the TEST() functions within the unit test file. I assume this is because the gtest directory cannot be found. I also tried using cmake . to build but that results in a ton of untrackable errors with the gtest.h header file itself. Apparently it seems that building with cmake . can track the gtest.h header file while building with cmake3 . cannot. I still have no clue on how to solve this error.

Here is my CMakeLists.txt file:

CMAKE_MINIMUM_REQUIRED(VERSION 2.8)

ADD_SUBDIRECTORY(googletest)

SET(CMAKE_CXX_STANDARD 11)

ADD_EXECUTABLE(test
       unit_test.cpp
)

TARGET_LINK_LIBRARIES(test gtest)
TARGET_COMPILE_DEFINITIONS(test PRIVATE gtest_disable_pthreads=ON)