Build allows missing nvidia CUDA compiler to cause error at compile time

[brettviren]

It looks like it is possible for wcb to get all the way to building cuda/ subpackage while having an empty string set for the nvcc compiler.

Errors from wcb are like:

Exception: ['-shared', '-Xcompiler', '-fPIC', '-I.', '-Icuda/inc', '-I../cuda/inc', '../cuda/src/simplecudatest.cu', '-c', '-o', 'cuda/src/simplecudatest.cu.2.o']
...
FileNotFoundError: [Errno 2] No such file or directory: '-shared'

It is not clear if this is a failure in the waft/ detection of CUDA or something else.

If CUDA support is not required, the simplest work-around for now is to not build the cuda/ sub-package even if CUDA is (half) detected. This is done with the option:

./wcb configure --with-cuda=no [...nominal configure options...]