Error while cmake by building from zip-archive

* add validation is git found
* add validation is .git folder exists in project (project zip archive download without .git folder)
* get head commit SHA if git found and .git folder exists

Tested:
* configure project by CMake 3.0.0 successfully (with and without git), with project zip archive (without .git folder) and with project cloned from github
* configure project by command: cmake -G "Visual Studio 12 Win64"
* build solution by Visual Studio
* manually validate that file utils/build_version.cc contains valid head revision value

CMakeLists.txt

@@ -31,7 +31,14 @@ string(REGEX REPLACE "(..)/(..)/..(..).*" "\\1/\\2/\\3" DATE ${DATE})
 string(REGEX REPLACE "(..):(.....).*" " \\1:\\2" TIME ${TIME})
 string(CONCAT GIT_DATE_TIME ${DATE} ${TIME})
 
-execute_process(COMMAND $ENV{COMSPEC} " /C git -C ${CMAKE_CURRENT_SOURCE_DIR} rev-parse HEAD" OUTPUT_VARIABLE GIT_SHA)
+find_package(Git)
+
+if (GIT_FOUND AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/.git")
+    execute_process(COMMAND $ENV{COMSPEC} /C ${GIT_EXECUTABLE} -C ${CMAKE_CURRENT_SOURCE_DIR} rev-parse HEAD OUTPUT_VARIABLE GIT_SHA)
+else()
+    set(GIT_SHA 0)
+endif()
+
 string(REGEX REPLACE "[^0-9a-f]+" "" GIT_SHA ${GIT_SHA})
 
 set(BUILD_VERSION_CC ${CMAKE_CURRENT_SOURCE_DIR}/util/build_version.cc)