Do not install the bundled GoogleTest

GoogleTest is a test-only dependency, but installing Ceres built with
the GoogleTest submodule also installed its libraries, headers and
package configuration, possibly overwriting an existing installation.

Disable the GoogleTest install rules when embedding the submodule.

Change-Id: If2783ab65876d50efacffce03f40ef7416e07956

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 05fea16..da26b18 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -297,6 +297,10 @@
     # work.
 
     # set(GTEST_HAS_ABSL ON CACHE BOOL "Use ABSL when building gtest")
+
+    # GoogleTest is only needed for building the tests and must therefore not
+    # be installed alongside Ceres.
+    set(INSTALL_GTEST OFF)
     add_subdirectory(${Ceres_SOURCE_DIR}/third_party/googletest ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/googletest)
     set(GTest_FOUND TRUE)
     ceres_register_found_package(GTest)