perf tools: Use Python devtools for version autodetection rather than runtime

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: 79373082fa ("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.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile.config

@@ -239,18 +239,33 @@ ifdef PARSER_DEBUG
 endif
 
 # Try different combinations to accommodate systems that only have
-# python[2][-config] in weird combinations but always preferring
-# python2 and python2-config as per pep-0394. If python2 or python
-# aren't found, then python3 is used.
-PYTHON_AUTO := python
-PYTHON_AUTO := $(if $(call get-executable,python3),python3,$(PYTHON_AUTO))
-PYTHON_AUTO := $(if $(call get-executable,python),python,$(PYTHON_AUTO))
-PYTHON_AUTO := $(if $(call get-executable,python2),python2,$(PYTHON_AUTO))
-override PYTHON := $(call get-executable-or-default,PYTHON,$(PYTHON_AUTO))
-PYTHON_AUTO_CONFIG := \
-  $(if $(call get-executable,$(PYTHON)-config),$(PYTHON)-config,python-config)
-override PYTHON_CONFIG := \
-  $(call get-executable-or-default,PYTHON_CONFIG,$(PYTHON_AUTO_CONFIG))
+# python[2][3]-config in weird combinations in the following order of
+# priority from lowest to highest:
+#   * python3-config
+#   * python-config
+#   * python2-config as per pep-0394.
+#   * $(PYTHON)-config (If PYTHON is user supplied but PYTHON_CONFIG isn't)
+#
+PYTHON_AUTO := python-config
+PYTHON_AUTO := $(if $(call get-executable,python3-config),python3-config,$(PYTHON_AUTO))
+PYTHON_AUTO := $(if $(call get-executable,python-config),python-config,$(PYTHON_AUTO))
+PYTHON_AUTO := $(if $(call get-executable,python2-config),python2-config,$(PYTHON_AUTO))
+
+# If PYTHON is defined but PYTHON_CONFIG isn't, then take $(PYTHON)-config as if it was the user
+# supplied value for PYTHON_CONFIG. Because it's "user supplied", error out if it doesn't exist.
+ifdef PYTHON
+  ifndef PYTHON_CONFIG
+    PYTHON_CONFIG_AUTO := $(call get-executable,$(PYTHON)-config)
+    PYTHON_CONFIG := $(if $(PYTHON_CONFIG_AUTO),$(PYTHON_CONFIG_AUTO),\
+                          $(call $(error $(PYTHON)-config not found)))
+  endif
+endif
+
+# Select either auto detected python and python-config or use user supplied values if they are
+# 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)))
 
 grep-libs  = $(filter -l%,$(1))
 strip-libs  = $(filter-out -l%,$(1))