Renamed option `--with-gdb-check` to `--enable-gdb-check`.

* bootstrap:
  * Added the support of `--disable-gdb` command line option.
  * Renamed command line options `--with-gdb-check` and
    `--without-gdb-check` to `--enable-gdb-check` and `--disable-gdb-check`,
    respectively.
* jamroot:
  * Added the support of `--disable-gdb` command line option.
  * Only the last one of `--enable-gdb` and `--disable-gdb` command line
    options is effective.
  * Rename command line options `--with-gdb-check` and `--without-gdb-check`
    to `--enable-gdb-check` and `--disable-gdb-check`, respectively.
  * Only the last one of `--enable-gdb-check` and `--disable-gdb-check`
    command line options is effective.
Merge pull request #120 from Cryolite/fix/enableGdbCheck

Renamed option `--with-gdb-check` to `--enable-gdb-check`.