Add --disable-maintainer-mode; make bison optional

Also flex is now optional.

The outputs of flex and bison are now committed.  By default they get
built, but users who want to build from git can now

    ./configure --disable-maintainer-mode

to turn off the dependency on bison and flex.

Maintainers must, of course, commit the bison and/or flex outputs when
they make changes to parser.y and/or lexer.l, respectively.

configure.ac

@@ -12,6 +12,7 @@ AC_CONFIG_AUX_DIR([config])
 AM_INIT_AUTOMAKE([parallel-tests foreign -Wall])
 AM_SILENT_RULES([yes])
 AM_PROG_AR
+AM_MAINTAINER_MODE([enable])
 AC_PROG_CC
 AC_PROG_CC_STDC
 AC_PROG_CPP_WERROR
@@ -27,28 +28,20 @@ dnl AC_PROG_YACC macros...
 
 dnl check bison version
 
-if test "$YACC" != "bison -y"; then
-  AC_MSG_CHECKING([bison version])
-  AC_MSG_RESULT([not bison])
-else
-  AX_PROG_BISON_VERSION([3],
-    [],
-    [AC_MSG_ERROR([You need bison version 3.0 or greater.])])
-  dnl These gross square brackets keep autotools from eating the perl script...
-#  yacc_version=[$($YACC --version 2>1 | perl -pe '($_)=/([0-9]+([.][0-9]+)+)/')]
-#  AX_COMPARE_VERSION([$yacc_version], [gt], [3],
-#  [AC_MSG_RESULT([ok])],
-#  [AC_MSG_RESULT([no])
-#   AC_MSG_ERROR([You need bison version 3.0 or greater.])])
-fi
+if test "$USE_MAINTAINER_MODE" = yes; then
+  if test "$YACC" != "bison -y"; then
+    AC_MSG_CHECKING([bison version])
+    AC_MSG_RESULT([not bison])
+  else
+    AX_PROG_BISON_VERSION([3],
+      [],
+      [AC_MSG_ERROR([You need bison version 3.0 or greater.])])
+  fi
 
-dnl
-dnl these program checks should probably be deleted
-dnl
-
-AC_PROG_LEX
-if test "x$LEX" != xflex; then
-  LEX="$SHELL $missing_dir/missing flex"
+  AC_PROG_LEX
+  if test "x$LEX" != xflex; then
+    LEX="$SHELL $missing_dir/missing flex"
+  fi
 fi