We build different versions of the runtime to support different targets
and compiler options.  Each version has its own build directory, which is
created at configuration time.  The naming convention for these directories
is as follows:

    rt-<target>-<precision>[-par][-debug]

where the targets are

    c		- C with vector extensions
    cl		- C host + OpenCL code
    cuda	- C host + CUDA code

and precision is

    f		- single-precision
    d		- double-precision

We also allow "-par" to signify a parallel host runtime and "-debug" to
signify that the runtime was compiled with "-g" and assertions enabled.