Fix various APPLE only variables to include a prefix (and, the correct one).
Hack 'gnuradio-runtime' parameters into place before they are used. Use GNURADIO_RUNTIME in 'test-gnuradio-pmt', as well as use its correct sources.
Get COMPONENT dependencies correct when not WIN32 and not APPLE; GR_BLOCKS is always required, and this way should be more robust than before.
+ Set 'gr_fcd_extra_internal_include_dirs' in each subdirectory, and use it in GR_LIBRARY_FOO correctly to get the extra internal include directories.
+ do not use 'USB_INCLUDES' since it does not exist, and LIBUSB_INCLUDE_DIR is used in the proper location already (with the correct name).
+ LIBUSB_LIBRARY_DIRS does not exist, so don't use it.
+ use APPLE instead of matching on "Darwin".