Ignore generated src_win/BEViewerLauncher.exe.
* Updated CONFIGURE_F18.bash to build liblightgrep from the 1.2.0
release.

* Modified build_mingw function not to construct download URL from a
base URL and a tarball filename. Doing it this way is brittle, since the
filename might be set by a Content-Disposition header instead of by the
URL.

* Run bootstrap.sh only if configure doesn't exist.