Reduced the too many number of `wget` retrying.

* bootstrap: Added `-t 3` option to `wget` command because the default
             number of retring, which is equal to 20, is too many for the
             purpose of this project.
* util/make-srcdir.sh: Likewise.
* gcc/jamfile: Likewise.
* clang/jamfile: Likewise.
* icu4c/jamfile: Likewise.
* mpc/jamfile: Likewise.
* mpfr/jamfile: Likewise.
* opencv/jamfile: Likewise.
* openmpi/jamfile: Likewise.
* ppl/jamfile: Likewise.
* valgrind/jamfile: Likewise.