Add a regression test for m4(1).

This checks the behaviour of changecom() in both GNU and BSD modes of our m4.

tools/regression/usr.bin/m4/Makefile

@@ -0,0 +1,4 @@
+# $FreeBSD$
+
+all:
+	@sh ${.CURDIR}/regress.sh ${.CURDIR}

tools/regression/usr.bin/m4/regress.changecom.in

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+changecom
+# dnl BSD will show this in output.
+# dnl SYSV + GNU will not show this.

tools/regression/usr.bin/m4/regress.changecom.out

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+
+# dnl BSD will show this in output.
+# dnl SYSV + GNU will not show this.

tools/regression/usr.bin/m4/regress.gchangecom.out

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+
+# # 

tools/regression/usr.bin/m4/regress.sh

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+# $FreeBSD$
+
+# Go into the regression test directory, handed to us by make(1)
+TESTDIR=$1
+if [ -z "$TESTDIR" ]; then
+  TESTDIR=.
+fi
+cd $TESTDIR
+
+STATUS=0
+
+for test in GNU/changecom changecom; do
+  echo "Running test $test"
+  case "$test" in
+  GNU/*)
+    M4="m4 -g"
+    GNU="g"
+    test=`basename $test`
+    ;;
+  *)
+    M4="m4"
+    GNU=""
+    ;;
+  esac
+  case "$test" in
+  changecom)
+    $M4 < regress.$test.in | diff -u regress.$GNU$test.out -
+    ;;
+  esac
+  if [ $? -eq 0 ]; then
+    echo "PASS: Test $test detected no regression, output matches."
+  else
+    STATUS=$?
+    echo "FAIL: Test $test failed: regression detected.  See above."
+  fi
+done
+
+exit $STATUS