Deal with backported bool type for RHEL5's 2.6.18 kernel

Deal with backported bool type for RHEL5's 2.6.18 kernel

RHEL5's 2.6.18 kernel includes a change backported from 2.6.19 that
declares a generic 'bool' type in <linux/types.h> and declares the
'false' and 'true' enum values in <linux/stddef.h>.  Comedi's
compatibility header for <linux/stddef.h> declared the 'false' and
'true' enum values for any kernel version below 2.6.19.  This resulted
in comedi failing to build due to redeclaration errors.

Add an autoconf test to see if the generic 'bool' type is defined in
<linux/types.h> and if so define the macro
CONFIG_COMEDI_HAVE_GENERIC_BOOL_TYPE in <config.h>.  Conditionally
declare the 'bool' type in the <linux/types.h> compatibility header and
change the condition for declaring the 'false' and 'true' enum values in
the <linux/stddef.h> compatibility header so they are only declared if
the CONFIG_COMEDI_HAVE_GENERIC_BOOL_TYPE macro is undefined.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>