Added automake conditionals CONFIG_COMEDI_USB and CONFIG_COMEDI_PCMCIA and

Added automake conditionals CONFIG_COMEDI_USB and CONFIG_COMEDI_PCMCIA and
used these in place of the automake conditionals CONFIG_USB and
CONFIG_PCMCIA.  The automake conditionals CONFIG_USB and CONFIG_PCMCIA are
no longer used (currently) but they no longer depend on the --disable-usb
or --disable-pcmcia configure options.  The new automake conditional
CONFIG_COMEDI_USB/CONFIG_COMEDI_PCMCIA is 'FALSE' if --disable-usb/
--disable-pcmcia is specified; otherwise it has the same value as
CONFIG_USB/CONFIG_PCMCIA.

Added config.h defines CONFIG_COMEDI_USB and CONFIG_COMEDI_PCMCIA which are
defined only if the corresponding automake conditional is 'TRUE'.

Changed the --(en|dis)able-pcmcia option so that it is no longer necessary
to specify --enable-pcmcia explicitly to build the PCMCIA modules.