Extend "make shellcheck" onto mkdep too. [skip appveyor]

For shell trap purposes, "1 2 3 13 15" is the same as "HUP INT QUIT PIPE
TERM" in AIX, FreeBSD, Hurd, illumos, Linux, macOS, NetBSD, OpenBSD and
Solaris.  However, in Haiku SIGPIPE is 7 and 13 is SIGTSTP.

Address all warnings from shellcheck 0.9.0:

mkdep:66:11: note: Double quote to prevent globbing and word splitting.
  [SC2086]
mkdep:73:34: warning: Trapping signals by number is not well defined.
  Prefer signal names. [SC2172]
mkdep:75:4: note: Double quote to prevent globbing and word splitting.
  [SC2086]
mkdep:75:10: note: Double quote to prevent globbing and word splitting.
  [SC2086]
mkdep:77:41: note: Double quote to prevent globbing and word splitting.
  [SC2086]
mkdep:77:49: note: Double quote to prevent globbing and word splitting.
  [SC2086]
mkdep:79:17: note: Double quote to prevent globbing and word splitting.
  [SC2086]
mkdep:97:16: warning: Use "$@" (with quotes) to prevent whitespace problems.
  [SC2048]
mkdep:103:5: note: Double quote to prevent globbing and word splitting.
  [SC2086]
mkdep:103:23: note: Double quote to prevent globbing and word splitting.
  [SC2086]
mkdep:103:30: note: Double quote to prevent globbing and word splitting.
  [SC2086]
mkdep:106:11: note: Double quote to prevent globbing and word splitting.
  [SC2086]
mkdep:108:17: note: Double quote to prevent globbing and word splitting.
  [SC2086]
mkdep:114:4: note: Double quote to prevent globbing and word splitting.
  [SC2086]
mkdep:114:9: note: Double quote to prevent globbing and word splitting.
  [SC2086]
mkdep:115:7: note: Double quote to prevent globbing and word splitting.
  [SC2086]
mkdep:115:19: note: Double quote to prevent globbing and word splitting.
  [SC2086]
(cherry picked from commit 983d432)

Author: infrastation

Makefile.in

@@ -578,4 +578,4 @@ depend:
 	@$(MKDEP) -c $(CC) -m "$(DEPENDENCY_CFLAG)" -s "$(srcdir)" $(DEFS) $(INCLS) $(SRC) $(LIBNETDISSECT_SRC)
 
 shellcheck:
-	shellcheck -f gcc -e SC2006 autogen.sh build.sh build_matrix.sh build_common.sh .ci-coverity-scan-build.sh
+	shellcheck -f gcc -e SC2006 autogen.sh build.sh build_matrix.sh build_common.sh mkdep .ci-coverity-scan-build.sh

mkdep

@@ -63,20 +63,20 @@ if [ $# = 0 ] ; then
 	exit 1
 fi
 
-if [ ! -w $MAKE ]; then
+if [ ! -w "$MAKE" ]; then
 	echo "mkdep: no writeable file \"$MAKE\""
 	exit 1
 fi
 
 TMP=${TMPDIR:-/tmp}/mkdep$$
 
-trap 'rm -f $TMP ; exit 1' 1 2 3 13 15
+trap 'rm -f "$TMP" ; exit 1' HUP INT QUIT PIPE TERM
 
-cp $MAKE ${MAKE}.bak
+cp "$MAKE" "${MAKE}.bak"
 
-sed -e '/DO NOT DELETE THIS LINE/,$d' < $MAKE > $TMP
+sed -e '/DO NOT DELETE THIS LINE/,$d' < "$MAKE" > "$TMP"
 
-cat << _EOF_ >> $TMP
+cat << _EOF_ >> "$TMP"
 # DO NOT DELETE THIS LINE -- mkdep uses it.
 # DO NOT PUT ANYTHING AFTER THIS LINE, IT WILL GO AWAY.
 
@@ -94,23 +94,25 @@ _EOF_
 # Construct a list of source files with paths relative to the source directory.
 #
 sources=""
-for srcfile in $*
+for srcfile in "$@"
 do
 	sources="$sources $SOURCE_DIRECTORY/$srcfile"
 done
 
 # XXX this doesn't work with things like "-DDECLWAITSTATUS=union\ wait"
-$CC $DEPENDENCY_CFLAG $flags $sources |
+# $flags and $sources are meant to expand
+# shellcheck disable=SC2086
+"$CC" "$DEPENDENCY_CFLAG" $flags $sources |
 sed "
 	s; \./; ;g
-	$SED" >> $TMP
+	$SED" >> "$TMP"
 
-cat << _EOF_ >> $TMP
+cat << _EOF_ >> "$TMP"
 
 # IF YOU PUT ANYTHING HERE IT WILL GO AWAY
 _EOF_
 
 # copy to preserve permissions
-cp $TMP $MAKE
-rm -f ${MAKE}.bak $TMP
+cp "$TMP" "$MAKE"
+rm -f "${MAKE}.bak" "$TMP"
 exit 0