Make sure configure.py and ninja.cc always agree on if -t browse is included.

No behavior change on most platforms. On solaris, -t browse was compiled in
in ninja.cc but browse.cc wasn't compiled in, which probably means that building
on Solaris didn't work. It might be better now.

This also makes browse.cc automatically not included in bootstrap builds;
previously this was done manually through the NINJA_BOOTSTRAP check.
Merge pull request #852 from nico/browse

Make sure configure.py and ninja.cc always agree on if -t browse is incl...