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_benchmark.cmake

@@ -0,0 +1,32 @@
+# Some developers would be happier with the google benchmark 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(benchmark QUIET)
+if (benchmark_FOUND)
+  return()
+endif()
+
+option(FETCHCONTENT_UPDATES_DISCONNECTED TRUE)
+option(FETCHCONTENT_QUIET FALSE)
+include(FetchContent)
+
+FetchContent_Declare(googlebenchmark
+  GIT_REPOSITORY "https://github.com/google/benchmark"
+  GIT_TAG        62937f91b5c763a8e119d0c20c67b87bde8eff1c 
+  GIT_PROGRESS   TRUE
+)
+
+FetchContent_GetProperties(googlebenchmark)
+set (BENCHMARK_ENABLE_GTEST_TESTS OFF CACHE INTERNAL "")
+set (BENCHMARK_ENABLE_TESTING OFF CACHE INTERNAL "")
+if(googlebenchmark_POPULATED)
+  return()
+endif()
+
+FetchContent_Populate(googlebenchmark)
+add_subdirectory(
+  ${googlebenchmark_SOURCE_DIR}
+  ${googlebenchmark_BINARY_DIR}
+  EXCLUDE_FROM_ALL
+)