WIP: Cleanup for disis munger / make flext/disis_munger build work again

As discussed on the mailing list. This took a while, since I wanted to test on all platforms before submitting. It works for me on (Arch) Linux, macOS and Windows. (The latter fails with an inno-setup error apparently due to !167 (merged) as I just reported, but the packages\win32_inno\build directory is looking good, so once the issue in !167 (merged) is fixed, it should work.)

I tried to make this as clean and unintrusive as possible, but it does require a separate build system (disis/Makefile.flext, disis/disis_munger/Makefile) because of the special build requirements. As discussed on the mailing list, I'm completely bypassing the flext build scripts which don't work in msys2. My Makefiles work on all platforms. This hooks into the existing build system in externals/Makefile by just adding the corresponding make invocations to the existing disis targets (rev. 37916145). The end result is that disis_munger~.{pd_linux,pd_darwin,dll} and disis_munger~-help.pd get added to the installed extra/disis directory.

Notes:

• On macOS/Darwin building and installing of these is currently disabled (rev. adc218ba) because of the crash bug in flext on macOS I mentioned on the mailing list. But I checked that it compiles and installs properly on macOS, too, so that once that crash bug is fixed, we just revert rev. adc218ba and it will be ready to go in on the Mac, too.

• The flext libraries and header files are not installed right now. (Were these in the pd-l2ork 1.0 package? I haven't checked yet.) But in any case they should be easy to add if so desired, we just need to figure out where they should go, then I can add them to the install target in Makefile.flext.

• flext adds quite a bit to the total compile time, especially on Windows running in a VM. Seems like a lot of hassle to just add this one external, but OTOH with this PR we now also have the infrastructure in place to easily port other flext-based externals if the need arises.