Use MPC_VERSION variable.
Move tarball version and location into gcc482.opt.
Add gcc473.opt, for building gcc 4.7.3.
Read DJGPP_PREFIX, ENABLE_LANGUAGES, OPT_FILE from environment variable.
Use MAKE_CHECK environment variable to run "make check" or not.
Build exe2coff.
Add directory for $OPT_FILE.
Delete partial downloaded file if curl failed.
Update README.md.