Use shared gtest instead of submodule

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 617e4b2..2857875 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -18,7 +18,6 @@ set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules/")
 include(GNUInstallDirs)
 
 project(leelaz)
-add_subdirectory(gtest EXCLUDE_FROM_ALL) # We don't want to install gtest, exclude it from `all`
 
 # Required Packages
 set(Boost_MIN_VERSION "1.58.0")
@@ -27,6 +26,7 @@ find_package(Boost 1.58.0 REQUIRED program_options filesystem)
 find_package(Threads REQUIRED)
 find_package(ZLIB REQUIRED)
 find_package(OpenCL REQUIRED)
+find_package(GTest REQUIRED)
 # We need OpenBLAS for now, because we make some specific
 # calls. Ideally we'd use OpenBLAS is possible and fall back to
 # not doing those calls if it's not present.
@@ -93,6 +93,7 @@ include_directories(${IncludePath})
 include_directories(${Boost_INCLUDE_DIRS})
 include_directories(${OpenCL_INCLUDE_DIRS})
 include_directories(${ZLIB_INCLUDE_DIRS})
+include_directories(${GTEST_INCLUDE_DIRS})
 
 if((UNIX AND NOT APPLE) OR WIN32)
     include_directories(${BLAS_INCLUDE_DIRS})
@@ -141,7 +142,8 @@ target_link_libraries(tests ${Boost_LIBRARIES})
 target_link_libraries(tests ${BLAS_LIBRARIES})
 target_link_libraries(tests ${OpenCL_LIBRARIES})
 target_link_libraries(tests ${ZLIB_LIBRARIES})
-target_link_libraries(tests gtest_main ${CMAKE_THREAD_LIBS_INIT})
+target_link_libraries(tests ${GTEST_BOTH_LIBRARIES})
+target_link_libraries(tests ${CMAKE_THREAD_LIBS_INIT})
 
 include(GetGitRevisionDescription)
 git_describe(VERSION --tags)
d='get' action='/guix/log/tests/guix-build-branch.sh'>
<input type='hidden' name='id' value='3887c1f3b3ee679042387c444aa663f67172cc66'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=3887c1f3b3ee679042387c444aa663f67172cc66'>root</a>/<a href='/guix/log/tests?id=3887c1f3b3ee679042387c444aa663f67172cc66'>tests</a>/<a href='/guix/log/tests/guix-build-branch.sh?id=3887c1f3b3ee679042387c444aa663f67172cc66'>guix-build-branch.sh</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/guix-build-branch.sh?id=3887c1f3b3ee679042387c444aa663f67172cc66&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-09-28 21:06:36 -0500'>2020-09-28</span></td><td><a href='/guix/commit/tests/guix-build-branch.sh?id=d8934360d2453a403b5433e71d09188e4ed23b57'>tests: Simplify shell exit status negation;</a><span class='msg-avail'>...</span></td><td>Eric Bavier</td></tr>
<tr><td><span title='2020-04-04 18:52:36 +0200'>2020-04-04</span></td><td><a href='/guix/commit/tests/guix-build-branch.sh?id=3cea55078857ecd8ca67fd7cf4eaebb13fb6d9d3'>tests: Adjust '--with-commit' test for tags.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-09-04 13:02:27 +0200'>2019-09-04</span></td><td><a href='/guix/commit/tests/guix-build-branch.sh?id=3c82f1254116be2a9216b7c7e5e8c001ff486270'>tests: Adjust '--with-commit' test.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-07-26 11:21:36 +0200'>2019-07-26</span></td><td><a href='/guix/commit/tests/guix-build-branch.sh?id=10a8c2bbc6754bddb0e5d668b3ecc9050efd820b'>git: 'switch-to-ref' resolves tag targets.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-07-26 11:21:36 +0200'>2019-07-26</span></td><td><a href='/guix/commit/tests/guix-build-branch.sh?id=177fecb57c0c9e15249bf6a49244c9dc6eb8439c'>git: &lt;git-checkout&gt; allows tags in its 'commit' field.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-11-30 17:03:04 +0100'>2018-11-30</span></td><td><a href='/guix/commit/tests/guix-build-branch.sh?id=a3d77c51bc8f641c12989e18dc5e03add776f87e'>git: Nicely report '--with-commit' errors.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-11-30 17:03:04 +0100'>2018-11-30</span></td><td><a href='/guix/commit/tests/guix-build-branch.sh?id=b18f7234aac9eb42097c1b4cda7efe0be5aab132'>guix build: Add '--with-commit'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-11-30 17:03:04 +0100'>2018-11-30</span></td><td><a href='/guix/commit/tests/guix-build-branch.sh?id=96915a448cfe8383a1c47f4b9a1cc810e5161fd0'>guix build: Add '--with-branch' transformation option.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>