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)
right' method='get' action='/guix/log/gnu/tests/virtualization.scm'>
<input type='hidden' name='id' value='b8085fccf2289fb3c64f21d6c5f811afe00fcedf'/><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=b8085fccf2289fb3c64f21d6c5f811afe00fcedf'>root</a>/<a href='/guix/log/gnu?id=b8085fccf2289fb3c64f21d6c5f811afe00fcedf'>gnu</a>/<a href='/guix/log/gnu/tests?id=b8085fccf2289fb3c64f21d6c5f811afe00fcedf'>tests</a>/<a href='/guix/log/gnu/tests/virtualization.scm?id=b8085fccf2289fb3c64f21d6c5f811afe00fcedf'>virtualization.scm</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/gnu/tests/virtualization.scm?id=b8085fccf2289fb3c64f21d6c5f811afe00fcedf&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-10-01 15:47:29 +0200'>2020-10-01</span></td><td><a href='/guix/commit/gnu/tests/virtualization.scm?id=18e76f89055f25f015fadb7c999b410f38a88cc6'>services: hurd-vm: Resurrect system-test by using raw disk-image.</a><span class='msg-avail'>...</span></td><td>Jan (janneke) Nieuwenhuizen</td></tr>
<tr><td><span title='2020-09-29 21:56:28 +0200'>2020-09-29</span></td><td><a href='/guix/commit/gnu/tests/virtualization.scm?id=c11c19bd4d0dc4ec56b949647057dbf00567f2ae'>services: hurd-vm: Add system test.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>