This adds missing includes for alloca. The man page of alloca(3) states
that alloca.h should be #include'd to use alloca.

Also work around missing alloca.h header on Windows by adding a
compatibility module.

Closes: #197

This commit introduces the skeleton needed to implement reusable
operating system compatibility modules. This skeleton can then be
extended with the actual compatibility modules that work around
operating system specifics.

For example this will be used to be able to compile files that use
alloca on Windows. Also it can be used to implement replacements for
functions that are missing on some systems.