Makefile.in: Add the shellcheck target

Run shellcheck on the build.sh and the build_matrix.sh scripts.
(using 'gcc' format: GCC compatible output. See the shellcheck man page)

[skip ci]

(cherry picked from commit 657d672)

Author: fxlb

Makefile.in

@@ -475,3 +475,6 @@ testlist:
 
 depend:
 	$(MKDEP) -c "$(CC)" -m "$(DEPENDENCY_CFLAG)" -s "$(srcdir)" $(DEFS) $(INCLS) $(SRC) $(LIBNETDISSECT_SRC)
+
+shellcheck:
+	shellcheck -f gcc build.sh build_matrix.sh