Avoid string CONCAT which is not supported in cmake 2.6 (#1383)

Signed-off-by: Bassam Tabbara <bassam.tabbara@quantum.com>

CMakeLists.txt

@@ -58,11 +58,11 @@ if(WIN32)
   execute_process(COMMAND powershell -Command "Get-Date -format HH:mm:ss" OUTPUT_VARIABLE TIME)
   string(REGEX REPLACE "(..)_(..)_..(..).*" "\\1/\\2/\\3" DATE "${DATE}")
   string(REGEX REPLACE "(..):(.....).*" " \\1:\\2" TIME "${TIME}")
-  string(CONCAT GIT_DATE_TIME ${DATE} ${TIME})
+  set(GIT_DATE_TIME "${DATE} ${TIME}")
 else()
   execute_process(COMMAND date "+%Y/%m/%d %H:%M:%S" OUTPUT_VARIABLE DATETIME)
   string(REGEX REPLACE "\n" "" DATETIME ${DATETIME})
-  string(CONCAT GIT_DATE_TIME ${DATETIME})
+  set(GIT_DATE_TIME "${DATETIME}")
 endif()
 
 find_package(Git)