[fix] re-add 'pip-exe' target - partial revert 9b48ae47

Target pip-exe is a prerequisite of the targets:

  - pyinstall
  - pyuninstall

and was accidentally deleted in commit 9b48ae47.

HINT:
  do not confuse pyinstall with penvinstall

pyinstall & pyuninstall
    Installing into user's HOME using pip from OS,
    therefore the message is needed.

pyenvinstall & pyenvuninstall
    Installing into virtualenv (./local) using pip which is provided by
    prerequisite 'pyenv' in the virtualenv.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

utils/makefile.python

@@ -87,6 +87,22 @@ python-exe:
 	@:
 endif
 
+msg-pip-exe:
+	@echo "\n  $(PIP) is required\n\n\
+  Make sure you have updated pip installed, grab it from\n\
+  https://pip.pypa.io or install it from your package\n\
+  manager. On debian based OS these requirements are\n\
+  installed by::\n\n\
+    sudo -H apt-get install python$(PY)-pip\n" | $(FMT)
+
+ifeq ($(shell which $(PIP) >/dev/null 2>&1; echo $$?), 1)
+pip-exe: msg-pip-exe
+	$(error The '$(PIP)' command was not found)
+else
+pip-exe:
+	@:
+endif
+
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 # commands
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