if not yacc or bison is found, check if we *really* need it, and if we do

we bail out!

configure.ac

@@ -1144,6 +1144,20 @@ fi
 
 AC_PROG_YACC
 
+if test -z "$YACC"
+then
+  AC_MSG_CHECKING([if OK to build without bison/yacc])
+  dnl no yacc is a big deal if we have no pre-fixed getdate.y
+  if test -r "$srcdir/lib/getdate.c"
+  then
+    dnl all is well, we don't have to generate it!
+    AC_MSG_RESULT(yes)
+  else
+    AC_MSG_RESULT(no)
+    AC_MSG_ERROR([no yacc or bison found, can't build libcurl!])
+  fi
+fi
+
 dnl *************************************************************************
 dnl If the manual variable still is set, then we go with providing a built-in
 dnl manual