perf build: Fix in-tree build due to symbolic link

perf build: Fix in-tree build due to symbolic link

Building perf in-tree is broken after commit 890a1961c812 ("perf tools:
Create source symlink in perf object dir") which added a 'source' symlink
in the output dir pointing to the source dir.

With in-tree builds, the added 'SOURCE = ...' line is executed multiple
times (I observed 2 during the build plus 2 during installation). This is a
minor inefficiency, in theory not harmful because symlink creation is
assumed to be idempotent. But it is not.

Considering with in-tree builds:

  srctree=/absolute/path/to/linux
   OUTPUT=/absolute/path/to/linux/tools/perf

here's what happens:

 1. ln -sf $(srctree)/tools/perf $(OUTPUT)/source
    -> creates /absolute/path/to/linux/tools/perf/source
       link to /absolute/path/to/linux/tools/perf
    => OK, that's what was intended
 2. ln -sf $(srctree)/tools/perf $(OUTPUT)/source   # same command as 1
    -> creates /absolute/path/to/linux/tools/perf/perf
       link to /absolute/path/to/linux/tools/perf
    => Not what was intended, not idempotent
 3. Now the build _should_ create the 'perf' executable, but it fails

The reason is the tricky 'ln' command line. At the first invocation 'ln'
uses the 1st form:

       ln [OPTION]... [-T] TARGET LINK_NAME

and creates a link to TARGET *called LINK_NAME*.

At the second invocation $(OUTPUT)/source exists, so 'ln' uses the 3rd
form:

       ln [OPTION]... TARGET... DIRECTORY

and creates a link to TARGET *called TARGET* inside DIRECTORY.

Fix by adding -n/--no-dereference to "treat LINK_NAME as a normal file
if it is a symbolic link to a directory", as the manpage says.

Closes: https://lore.kernel.org/all/20241125182506.38af9907@booty/
Fixes: 890a1961c812 ("perf tools: Create source symlink in perf object dir")
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Tested-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20250124-perf-fix-intree-build-v1-1-485dd7a855e4@bootlin.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>