perf python: Fix build when PYTHON_CONFIG is user supplied

The previous change to Python autodetection had a small mistake where
the auto value was used to determine the Python binary, rather than the
user supplied value. The Python binary is only used for one part of the
build process, rather than the final linking, so it was producing
correct builds in most scenarios, especially when the auto detected
value matched what the user wanted, or the system only had a valid set
of Pythons.

Change it so that the Python binary path is derived from either the
PYTHON_CONFIG value or PYTHON value, depending on what is specified by
the user. This was the original intention.

This error was spotted in a build failure an odd cross compilation
environment after commit 4c41cb4 ("perf python: Prefer
python3") was merged.

Fixes: 630af16 ("perf tools: Use Python devtools for version autodetection rather than runtime")
Signed-off-by: James Clark <[email protected]>
Acked-by: Ian Rogers <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: James-A-Clark

tools/perf/Makefile.config

@@ -265,7 +265,7 @@ endif
 # defined. get-executable-or-default fails with an error if the first argument is supplied but
 # doesn't exist.
 override PYTHON_CONFIG := $(call get-executable-or-default,PYTHON_CONFIG,$(PYTHON_AUTO))
-override PYTHON := $(call get-executable-or-default,PYTHON,$(subst -config,,$(PYTHON_AUTO)))
+override PYTHON := $(call get-executable-or-default,PYTHON,$(subst -config,,$(PYTHON_CONFIG)))
 
 grep-libs  = $(filter -l%,$(1))
 strip-libs  = $(filter-out -l%,$(1))