This commit implements support for using nested build directories of
the form

    (o) build/sequential/debug/cmake/clang
    (o) build/parallel/release/autotools/gcc

in addition to the separate build directory feature that already
existed.  This enables confining multiple build configurations to a
single subdirectory of each module's directory to avoid otherwise
littering the module directory.

Note: Using "mkdir -p" presumes a SUS v3 mkdir(1) utility.