Fix CMake when git is not found.

Fix: https://github.com/ArthurSonzogni/FTXUI/issues/62

CMakeLists.txt

@@ -1,11 +1,15 @@
 cmake_minimum_required(VERSION 3.0)
 
-execute_process(
+set(git_version 0)
+find_package(git QUIET)
+if (git_FOUND)
+  execute_process(
   COMMAND git rev-list --count HEAD
-  WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
+  WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
   OUTPUT_VARIABLE git_version
   OUTPUT_STRIP_TRAILING_WHITESPACE
 )
+endif()
 
 project(ftxui
   LANGUAGES CXX