use cmake TIMESTAMP if available (to help build reproducibility)

CMakeLists.txt

@@ -169,8 +169,12 @@ if(IAMCHILD)
   set(GMSH_API ${GMSH_API} PARENT_SCOPE)
 endif(IAMCHILD)
 
+if(${CMAKE_MAJOR_VERSION} GREATER 2)
+  string(TIMESTAMP DATE "%Y%m%d")
+else(${CMAKE_MAJOR_VERSION} GREATER 2)
   execute_process(COMMAND date "+%Y%m%d" OUTPUT_VARIABLE DATE
                   OUTPUT_STRIP_TRAILING_WHITESPACE)
+endif(${CMAKE_MAJOR_VERSION} GREATER 2)
 execute_process(COMMAND hostname OUTPUT_VARIABLE HOSTNAME
                 OUTPUT_STRIP_TRAILING_WHITESPACE)
 execute_process(COMMAND whoami OUTPUT_VARIABLE PACKAGER