Update googletest to 1.13

Fix issue with CMake 4.0

test/googletest-1.13.0/docs/pkgconfig.md

@@ -0,0 +1,148 @@
+## Using GoogleTest from various build systems
+
+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
+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>