Better alloca() support under Win32

[Tollef Fog Heen]

]] "carlo.bramix" 

| I would like to suggest this fix for pkg-config.
| I added the test on malloc.h into the configure script and I also added the code into some sources for including this file if it is found.
| This is a recommended action to do when compiling under Win32 because it will allow to recognize the alloca() function as intrinsic (it's better and it removes the warnings at compile time).

Thanks, applied to the development branch.

(And lots of apologies from me for taking forever to get this applied.)

-- 
Tollef Fog Heen
UNIX is user friendly, it's just picky about who its friends are