Change build date to git date (#5949)

Summary:
**Summary**
Replace in the version string the build date with the date of the last git commit.
The version should depend on the commits and not on the build date (building same code but on different dates should provide the same version string).

Pull Request resolved: #5949

Reviewed By: siyan-lin

Differential Revision: D36259342

Pulled By: khabinov

fbshipit-source-id: 50d1177e63b3041c176dfa117c0b3c292323e94e

Author: mciprian13

CMakeLists.txt

@@ -110,7 +110,7 @@ add_definitions("-DGIT_TAG=\"${GIT_TAG}\"")
 # Compile definition for Glow build date in Y-M-D format.
 string(TIMESTAMP GLOW_BUILD_DATE "%Y-%m-%d")
 add_definitions("-DGLOW_BUILD_DATE=\"${GLOW_BUILD_DATE}\"")
-add_definitions("-DGLOW_VERSION=\"${GLOW_BUILD_DATE} (${GIT_SHA1}) (${GIT_TAG})\"")
+add_definitions("-DGLOW_VERSION=\"${GIT_DATE} (${GIT_SHA1}) (${GIT_TAG})\"")
 
 if(GLOW_USE_PNG_IF_REQUIRED)
   find_package(PNG)