This fixes the issue where the build will fail if only the Python2
runtime is installed but the Python3 devtools are installed. Currently
the workaround is 'make PYTHON=python3'.

Fix it by autodetecting Python based on whether python[x]-config exists
rather than just python[x] because both are needed for the build. Then
-config is stripped to find the Python runtime.

Testing
=======

 * Auto detect links with Python3 when the v3 devtools are installed
   and only Python 2 runtime is installed
 * Auto detect links with Python2 when both devtools are installed
 * Sensible warning is printed if no Python devtools are installed
 * 'make PYTHON=x' still automatically sets PYTHON_CONFIG=x-config
 * 'make PYTHON=x' fails if x-config doesn't exist
 * 'make PYTHON=python3' overrides Python2 devtools
 * 'make PYTHON=python2' overrides Python3 devtools
 * 'make PYTHON_CONFIG=x-config' works
 * 'make PYTHON=x PYTHON_CONFIG=x' works
 * 'make PYTHON=missing' reports an error
 * 'make PYTHON_CONFIG=missing' reports an error

Fixes: 79373082 ("perf python: Autodetect python3 binary")
Signed-off-by: James Clark <james.clark@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20220309194313.3350126-2-james.clark@arm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>