cmake: Add Git module

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

cmake/Git.cmake

+find_package(Git)
+
+if(NOT GIT_FOUND OR NOT EXISTS "${PROJECT_SOURCE_DIR}/.git")
+    return()
+endif()
+
+# Update submodules as needed
+option(GIT_SUBMODULE "Check submodules during build" ON)
+if(NOT GIT_SUBMODULE)
+    return()
+endif()
+
+message(STATUS "Submodule update")
+execute_process(COMMAND ${GIT_EXECUTABLE} submodule update --init --recursive
+                WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
+                RESULT_VARIABLE GIT_SUBMODULE_RESULT)
+if(NOT GIT_SUBMODULE_RESULT EQUAL "0")
+    message(FATAL_ERROR "git submodule update --init failed with ${GIT_SUBMODULE_RESULT}, please checkout submodules")
+endif()
+
+# Fetch the necessary git variables
+execute_process(COMMAND ${GIT_EXECUTABLE} describe --always --tags
+                WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
+                OUTPUT_VARIABLE GIT_VERSION
+                OUTPUT_STRIP_TRAILING_WHITESPACE)