Make the build-time show up in the leveldb library.

Summary:
This is a regression caused by
772f75b3fb

If you do "strings libleveldb.a | grep leveldb_build_git_datetime" it will
show you the time when the binary was built.

Test Plan: make check

Reviewers: emayanke

Reviewed By: emayanke

CC: leveldb

Differential Revision: https://reviews.facebook.net/D9273

build_detect_version

@@ -20,6 +20,7 @@ else
   echo "git not found"| awk ' BEGIN {print "#include \"build_version.h\""} {print "const char * leveldb_build_git_sha = \"leveldb_build_git_sha:git not found\";"} END {}' > ${VFILE}
 fi
 
+date | awk 'BEGIN {} {print "const char * leveldb_build_git_datetime = \"leveldb_build_git_datetime:"$0"\";"} END {} ' >> ${VFILE}
 echo "const char * leveldb_build_compile_date = __DATE__;" >> ${VFILE}
 echo "const char * leveldb_build_compile_time = __TIME__;" >> ${VFILE}