Update Google Test to v1.10.0 (#840)

This commit updates the version of Google Test from 1.8 to 1.10.

test/gtest-1.10.0/googletest/docs/pkgconfig.md

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+## 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`.
+
+### Autotools
+
+Finding GoogleTest in Autoconf and using it from Automake is also fairly easy:
+
+In your `configure.ac`:
+
+```
+AC_PREREQ([2.69])
+AC_INIT([my_gtest_pkgconfig], [0.0.1])
+AC_CONFIG_SRCDIR([samples/sample3_unittest.cc])
+AC_PROG_CXX
+
+PKG_CHECK_MODULES([GTEST], [gtest_main])
+
+AM_INIT_AUTOMAKE([foreign subdir-objects])
+AC_CONFIG_FILES([Makefile])
+AC_OUTPUT
+```
+
+and in your `Makefile.am`:
+
+```
+check_PROGRAMS = testapp
+TESTS = $(check_PROGRAMS)
+
+testapp_SOURCES = samples/sample3_unittest.cc
+testapp_CXXFLAGS = $(GTEST_CFLAGS)
+testapp_LDADD = $(GTEST_LIBS)
+```
+
+### Meson
+
+Meson natively uses pkgconfig to query dependencies:
+
+```
+project('my_gtest_pkgconfig', 'cpp', version : '0.0.1')
+
+gtest_dep = dependency('gtest_main')
+
+testapp = executable(
+  'testapp',
+  files(['samples/sample3_unittest.cc']),
+  dependencies : gtest_dep,
+  install : false)
+
+test('first_and_only_test', testapp)
+```
+
+### Plain Makefiles
+
+Since `pkg-config` is a small Unix command-line utility, it can be used in
+handwritten `Makefile`s too:
+
+```makefile
+GTEST_CFLAGS = `pkg-config --cflags gtest_main`
+GTEST_LIBS = `pkg-config --libs gtest_main`
+
+.PHONY: tests all
+
+tests: all
+  ./testapp
+
+all: testapp
+
+testapp: testapp.o
+  $(CXX) $(CXXFLAGS) $(LDFLAGS) $< -o $@ $(GTEST_LIBS)
+
+testapp.o: samples/sample3_unittest.cc
+  $(CXX) $(CPPFLAGS) $(CXXFLAGS) $< -c -o $@ $(GTEST_CFLAGS)
+```
+
+### 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`.