[PATCH] build: correct cppcheck-misra make rule

[Jan Beulich <jbeulich@xxxxxxxx>]

It has been bothering me for a while that I made a bad suggestion during
review: Having cppcheck-misra.json depend on cppcheck-misra.txt does not
properly address the multiple targets problem. If cppcheck-misra.json
is deleted from the build tree but cppcheck-misra.txt is still there,
nothing will re-generate cppcheck-misra.json.

With GNU make 4.3 or newer we could use the &: grouped target separator,
but since we support older make as well we need to use some other
mechanism. Convert the rule to a pattern one (with "cppcheck"
arbitrarily chosen as the stem), thus making known to make that both
files are created by a single command invocation. Since, as a result,
the JSON file is now "intermediate" from make's perspective, prevent it
being deleted again by making it a prereq of .PRECIOUS.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
I've not been able to spot where / how cppcheck-misra.txt is used. If
it's indeed unused, a perhaps better alternative would be to convert the
original rule to specify cppcheck-misra.json as (the only) target. One
might then even consider using "-o /dev/null" instead of producing an
unused *.txt file.

--- a/xen/Makefile
+++ b/xen/Makefile
@@ -746,11 +746,9 @@ cppcheck-version:
 # documentation file. Also generate a json file with the right arguments for
 # cppcheck in json format including the list of rules to ignore.
 #
-cppcheck-misra.txt: $(XEN_ROOT)/docs/misra/rules.rst 
$(srctree)/tools/convert_misra_doc.py
-       $(Q)$(PYTHON) $(srctree)/tools/convert_misra_doc.py -i $< -o $@ -j 
$(@:.txt=.json)
-
-# convert_misra_doc is generating both files.
-cppcheck-misra.json: cppcheck-misra.txt
+.PRECIOUS: %-misra.json
+%-misra.txt %-misra.json: $(XEN_ROOT)/docs/misra/rules.rst 
$(srctree)/tools/convert_misra_doc.py
+       $(Q)$(PYTHON) $(srctree)/tools/convert_misra_doc.py -i $< -o 
$*-misra.txt -j $*-misra.json
 
 # Put this in generated headers this way it is cleaned by include/Makefile
 $(objtree)/include/generated/compiler-def.h: