record the timestamp on first configure (#4799)

Summary:
cmake doesn't re-generate the timestamp on subsequent builds causing rebuilds of the lib

This improves compile time turn-arounds if you have rocksdb as a compileable library include, since with the state its now it will re-generate the time stamp .cc file each time you build, and thus re-compile + re-link the rocksdb library though anything in the source actually changed.
The original timestamp is recorded into `CMakeCache.txt` and will remain there until you flush this cache.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4799

Differential Revision: D17290040

fbshipit-source-id: 28357fef3422693c9c19e88fa2873c8db0f662ed

CMakeLists.txt

@@ -135,7 +135,8 @@ else()
   endif()
 endif()
 
-string(TIMESTAMP GIT_DATE_TIME "%Y/%m/%d %H:%M:%S" UTC)
+string(TIMESTAMP TS "%Y/%m/%d %H:%M:%S" UTC)
+set(GIT_DATE_TIME "${TS}" CACHE STRING "the time we first built rocksdb")
 
 find_package(Git)