Changes

Building.md updated to reflect new changes in build process
As the recent changes in OMR made the build process move to `cmake`
usage, the current website shows the older build process which uses
`run_configure.mk` which might not be valid in future. It would also be
useful to have detailed process of Cmake which is documented in this
[file]
(https://github.com/eclipse/omr/blob/master/doc/BuildingWithCMake.md)
Signed-off-by: bharathappali <bharath.appali@gmail.com>