mingw: activate alloca

Both MSVC and MINGW have alloca(3) definitions in malloc.h, so by moving
win32-compat alloca.h from compat/vcbuild/include/ to compat/win32/ ,
which is included by both MSVC and MINGW CFLAGS, we can make alloca()
work on both those Windows environments.

In MINGW, malloc.h has explicit check for GNUC and if it is so, defines
alloca to __builtin_alloca, so it looks like we don't need to add any
code to here-shipped alloca.h to get optimum performance.

Compile-tested on Windows in MSysGit.

Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Acked-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.mak.uname

@@ -480,6 +480,7 @@ ifeq ($(uname_S),NONSTOP_KERNEL)
 endif
 ifneq (,$(findstring MINGW,$(uname_S)))
 	pathsep = ;
+	HAVE_ALLOCA_H = YesPlease
 	NO_PREAD = YesPlease
 	NEEDS_CRYPTO_WITH_SSL = YesPlease
 	NO_LIBGEN_H = YesPlease