Start implementing the build system.

Upon the quasi-completion of onchange, started writing the build system.
The build system is handwritten, and it will be able to do builds on
*BSD and Linuxen upon completion.
scripts/configure.sh: Enhance check_*_exists functions.

    command 1>/dev/null 2>/dev/null
    if [ $? ]; ...

is a better pattern to use with sh (1).