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

examples/CMakeLists.txt

@@ -1,3 +1,7 @@
+if(NOT FTXUI_BUILD_EXAMPLES)
+  return()
+endif()
+
 set(EXAMPLES_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 function(example name)
   add_executable(ftxui_example_${name} ${name}.cpp)