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

* bootstrap: Renamed command line options `--with-gcc-check` and
             `--without-gcc-check` to `--enable-gcc-check` and
             `--disable-gcc-check`, respectively.
* jamroot:
  * Renamed command line options `--with-gcc-check` and
    `--without-gcc-check` to `--enable-gcc-check` and `--disable-gcc-check`,
    respectively.
  * Only the last one of `--enable-gcc-check` and `--disable-gcc-check`
    command line options is effective.
Merge pull request #119 from Cryolite/fix/enableGcc

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