LU-19013 lnet: fix wording for GDS configure check

LU-19013 lnet: fix wording for GDS configure check

The wording on the GDS/CUDA configuration options is
incorrect. If the user does not specify external headers,
Lustre will fallback to the embedded headers rather
than disabling GDS.

Fix the wording on the configure options, improve the
macro name, and reorganize the header such that correct
defintions are under the ifdef.

Fixes: c65eabc2b113 ("LU-15189 build: add GDS configure options")
Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I645bc1c0c4bf26bdb9841c849b6cf8eebdc0bdee
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/59215
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>