Fix build on macos

[AkiSakurai]

1. times on macos do not accept NULL
2. There is system macro "howmany" in macos header, rename the function to avoid conflicts
3. Add missing EXTRA_CPPFLAGS to some makefiles
4. There is no strchrnul on macos
5. Fix configure script to forward arguments correctly
6. Do not link laxkit in plugin

[tomlechner]

Wow, you are going deep, my build system needs the attention!
Building for a mac has been requested many times, so this is looking very good.

I merged your companion Laxkit pr Laidout/laxkit#8.
Some minor notes for this pr:

1. Line 570 in src/interfaces/pagerangeinterface.cc should probably be the following, since part might be nullptr. Never crashed on me before since the old faulty part>0, while not doing what intended, didn't segfault!
   if (pos >= 0 || r >= 0 || (part && *part > 0)) return 1;

2. plugins/addonactions.o should be grouped with plugins/plugin.o in src/Makefile.

3. The AddonAction class is not really used yet, I think I accidentally last pushed without defining all the functions. It is not supposed to be an abstract class. I have some unpushed changes not quite ready for pushing that defines it a little more. It's ok to leave as is, it doesn't break anything, and I can adjust after merging your pr.

4. I need to fix the travis pull error, I probably need to update tokens. I've been meaning to revamp CI related things before the next Laidout version, so I've opened a new issue Revamp ci #27 if you are interested.

If you can adjust the pr for 1 and 2, then I can merge.

[tomlechner]

Looks good, thank you!