GoogleTest comes with pkg-config files that can be used to determine all necessary flags for compiling and linking to GoogleTest (and GoogleMock). Pkg-config is a standardised plain-text format containing

the includedir (-I) path
necessary macro (-D) definitions
further required flags (-pthread)
the library (-L) path
the library (-l) to link to

All current build systems support pkg-config in one way or another. For all examples here we assume you want to compile the sample samples/sample3_unittest.cc.

CMake

Using pkg-config in CMake is fairly easy:

cmake_minimum_required(VERSION 3.0)
 
cmake_policy(SET CMP0048 NEW)
project(my_gtest_pkgconfig VERSION 0.0.1 LANGUAGES CXX)
 
find_package(PkgConfig)
pkg_search_module(GTEST REQUIRED gtest_main)
 
add_executable(testapp samples/sample3_unittest.cc)
target_link_libraries(testapp ${GTEST_LDFLAGS})
target_compile_options(testapp PUBLIC ${GTEST_CFLAGS})
 
include(CTest)
add_test(first_and_only_test testapp)

It is generally recommended that you use target_compile_options + _CFLAGS over target_include_directories + _INCLUDE_DIRS as the former includes not just -I flags (GoogleTest might require a macro indicating to internal headers that all libraries have been compiled with threading enabled. In addition, GoogleTest might also require -pthread in the compiling step, and as such splitting the pkg-config Cflags variable into include dirs and macros for target_compile_definitions() might still miss this). The same recommendation goes for using _LDFLAGS over the more commonplace _LIBRARIES, which happens to discard -L flags and -pthread.

Help! pkg-config can't find GoogleTest!

Let's say you have a CMakeLists.txt along the lines of the one in this tutorial and you try to run cmake. It is very possible that you get a failure along the lines of:

-- Checking for one of the modules 'gtest_main'
CMake Error at /usr/share/cmake/Modules/FindPkgConfig.cmake:640 (message):
  None of the required 'gtest_main' found

These failures are common if you installed GoogleTest yourself and have not sourced it from a distro or other package manager. If so, you need to tell pkg-config where it can find the .pc files containing the information. Say you installed GoogleTest to /usr/local, then it might be that the .pc files are installed under /usr/local/lib64/pkgconfig. If you set

export PKG_CONFIG_PATH=/usr/local/lib64/pkgconfig

pkg-config will also try to look in PKG_CONFIG_PATH to find gtest_main.pc.

Using pkg-config in a cross-compilation setting

Pkg-config can be used in a cross-compilation setting too. To do this, let's assume the final prefix of the cross-compiled installation will be /usr, and your sysroot is /home/MYUSER/sysroot. Configure and install GTest using

mkdir build && cmake -DCMAKE_INSTALL_PREFIX=/usr ..

Install into the sysroot using DESTDIR:

make -j install DESTDIR=/home/MYUSER/sysroot

Before we continue, it is recommended to always define the following two variables for pkg-config in a cross-compilation setting:

export PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=yes

export PKG_CONFIG_ALLOW_SYSTEM_LIBS=yes

otherwise pkg-config will filter -I and -L flags against standard prefixes such as /usr (see https://bugs.freedesktop.org/show_bug.cgi?id=28264#c3 for reasons why this stripping needs to occur usually).

If you look at the generated pkg-config file, it will look something like

libdir=/usr/lib64
includedir=/usr/include
 
Name: gtest
Description: GoogleTest (without main() function)
Version: 1.11.0
URL: https://github.com/google/googletest
Libs: -L${libdir} -lgtest -lpthread
Cflags: -I${includedir} -DGTEST_HAS_PTHREAD=1 -lpthread

Notice that the sysroot is not included in libdir and includedir! If you try to run pkg-config with the correct PKG_CONFIG_LIBDIR=/home/MYUSER/sysroot/usr/lib64/pkgconfig against this .pc file, you will get

$ pkg-config --cflags gtest
-DGTEST_HAS_PTHREAD=1 -lpthread -I/usr/include
$ pkg-config --libs gtest
-L/usr/lib64 -lgtest -lpthread

which is obviously wrong and points to the CBUILD and not CHOST root. In order to use this in a cross-compilation setting, we need to tell pkg-config to inject the actual sysroot into -I and -L variables. Let us now tell pkg-config about the actual sysroot

export PKG_CONFIG_DIR=
export PKG_CONFIG_SYSROOT_DIR=/home/MYUSER/sysroot
export PKG_CONFIG_LIBDIR=${PKG_CONFIG_SYSROOT_DIR}/usr/lib64/pkgconfig

and running pkg-config again we get

$ pkg-config --cflags gtest
-DGTEST_HAS_PTHREAD=1 -lpthread -I/home/MYUSER/sysroot/usr/include
$ pkg-config --libs gtest
-L/home/MYUSER/sysroot/usr/lib64 -lgtest -lpthread

which contains the correct sysroot now. For a more comprehensive guide to also including ${CHOST} in build system calls, see the excellent tutorial by Diego Elio Pettenò: https://autotools.io/pkgconfig/cross-compiling.html