CMakeLists.txt: use external gtest, fallback to fetching the source

[barracuda156]

Resolves #287

[greg7mdp]

Thanks for the fix @barracuda156

[greg7mdp]

Hum, This fails to build on linux. I get build errors:

In file included from /home/greg/github/greg/gtl/tests/phmap/flat_hash_map_test.cpp:41:
/home/greg/github/greg/gtl/tests/phmap/unordered_map_constructor_test.hpp:28:10: fatal error: 'gmock/gmock.h' file not found
   28 | #include "gmock/gmock.h"
      |          ^~~~~~~~~~~~~~~
1 error generated.
[21/41] Building CXX object CMakeFiles/test_vector.dir/tests/misc/vector_test.cpp.o
ninja: build stopped: subcommand failed.

Looks like GTest_FOUND is true, but the target GTest::gmock_main is not present. On my system I have libgtest-dev installed.

[barracuda156]

@greg7mdp I cannot test on Linux, but maybe then add an explicit option instead? So that it can be turned off or on.

What does CMakeCache file has in the build directory? It is supposed to set paths to libs and headers.

[greg7mdp]

but maybe then add an explicit option instead? So that it can be turned off or on.

@barracuda156 8bd66dc