configure: Support toysh and *-redhat-linux target

[AlbydST]

Thank you for your time and effort on cproc!

I applied the following changes to the configure script:

• Fixed running with toysh (Toybox's built-in shell).
• Added support for the *-redhat-linux target (Used by Fedora & RHEL)
• Added a --help option to list all available options
• Added a --ignore-unknown-opts option to ignore unknown options
• When building for the host, the script now checks if the compiler, preprocessor, assembler, linker and qbe are installed. This can be turned off with the --skip-checks option.
• Added an end message which lists configuration options and their values. It also says if cproc is being cross-compiled or not.

I've verified that the script runs on coreutils, busybox, toybox and sbase to make sure it can run everywhere.

One thing I noticed while editing the script is that cproc does not have the -dumpmachine option which the script uses to determine the host.