Specialize paths to version.h in _vxworks-versions.h

Specialize paths to version.h in _vxworks-versions.h

The _vxworks-versions.h runtime file helps us control
the compilation of some library components depending on
the OS version extracted out of a system header.

The system header name is "version.h", and gcc has a
"version.h" file of its own.

gcc's version.h is now generated and the current
instance instead of the OS one, resulting in build failures
from

  #if !defined(_WRS_VXWORKS_MAJOR)
  #error "_WRS_VXWORKS_MAJOR undefined"
  #endif

This change introduces a twist in the way
_vxworks-versions.h #includes version.h, using a relative
path with components that will match in the OS include
dirs only.

The actual relative path is conditioned on _VSB_CONFIG_FILE
to accommodate a change in the include dirs organisation between
VxWorks 6 and 7.

2022-10-07  Olivier Hainque  <hainque@adacore.com>

gcc/
	* config/vxworks/_vxworks-versions.h: Use OS specific
	paths in #include of version.h.