PATCH: SYSROOT_SUBDIR_SPEC (was configure --with-sysroot)

[<gp at qnx dot com>]

I have attached my first attempt at support for SYSROOT_SUBDIR_SPEC and
SYSROOT_HEADERS_SUBDIR_SPEC.  I know this is not patches-gcc, but I am hoping
for some feedback before making that submission.

The suggestion I was given was to have SYSROOT_HEADERS_SUBDIR_SPEC default to
SYSROOT_SUBDIR_SPEC, and have SYSROOT_SUBDIR_SPEC default to an empty string.
 I chose to have them both default to empty strings.  It made sense to me to
keep them independant of each other, and to only use a subdir spec if it is
explicitly defined, and if target_system_root is set.

So we get:

 - target_system_root
   - headers and libs in sysroot.

 - target_system_root and target_sysroot_subdir
   - headers in sysroot, libs in subdir

 - target_system_root and target_sysroot_hdrs_subdir
   - headers found in hdrs_subdir, libs found in sysroot

 - target_system_root and both subdirs
   - headers in hdrs_subdir, libs in subdir

 - both subdirs
   - not used

PLMK.

Cheers.
GP

Attachment: gcc.c.diff
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