cmake: support gtest from the package manager (#552)

Some developers would be happier with the gtest version provided from
their package manager. Use it if it is installed the package provide
cmake support.

Fixed: https://github.com/ArthurSonzogni/FTXUI/issues/551

cmake/ftxui_find_google_test.cmake

@@ -0,0 +1,33 @@
+# Some developers would be happier with the gtest version provided from their
+# package manager. Use it if it is installed the package provide cmake support.
+# https://github.com/ArthurSonzogni/FTXUI/issues/551
+find_package(GTest QUIET)
+
+if (GTest_FOUND)
+  return()
+endif()
+
+option(FETCHCONTENT_UPDATES_DISCONNECTED TRUE)
+option(FETCHCONTENT_QUIET FALSE)
+include(FetchContent)
+
+FetchContent_Declare(googletest
+  GIT_REPOSITORY "https://github.com/google/googletest"
+  GIT_TAG        23ef29555ef4789f555f1ba8c51b4c52975f0907
+  GIT_PROGRESS   TRUE
+)
+
+FetchContent_GetProperties(googletest)
+if(googletest_POPULATED)
+  return()
+endif()
+
+FetchContent_Populate(googletest)
+set(BUILD_GMOCK OFF CACHE INTERNAL "")
+set(INSTALL_GTEST OFF CACHE INTERNAL "")
+set(gtest_force_shared_crt ON CACHE INTERNAL "")
+add_subdirectory(
+  ${googletest_SOURCE_DIR}
+  ${googletest_BINARY_DIR}
+  EXCLUDE_FROM_ALL
+)