autotools: Enable subdir-objects as many plugins use sub-directory objects

Many plugins build object files from sub-directories, and when doing
this it is better to enable Automake subdir-objects option.  So, enable
it, as it is good and old enough anyway.
autotools: Don't allow undefined symbols in plugins when possible

Tell the linker not to allow undefined symbols in built plugins it
possible.

This avoids use of non-plugin API functions from plugins as using those
would lead to undefined symbols.  This works even as we don't link to a
"libgeany" or the like because Geany plugin API is hidden behind macros
indirection.
Merge pull request #159 from b4n/autotools-various

Two Autotools improvements