kwset: allow building with GCC 8

The kwset functionality makes use of the obstack code, which expects to
be handed a function that can allocate large chunks of data. It expects
that function to accept a `size` parameter of type `long`.

This upsets GCC 8 on Windows, because `long` does not have the same
bit size as `size_t` there.

Now, the proper thing to do would be to switch to `size_t`. But this
would make us deviate from the "upstream" code even further, making it
hard to synchronize with newer versions, and also it would be quite
involved because that `long` type is so invasive in that code.

Let's punt, and instead provide a super small wrapper around
`xmalloc()`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

kwset.c

@@ -38,7 +38,13 @@
 #include "compat/obstack.h"
 
 #define NCHAR (UCHAR_MAX + 1)
-#define obstack_chunk_alloc xmalloc
+/* adapter for `xmalloc()`, which takes `size_t`, not `long` */
+static void *obstack_chunk_alloc(long size)
+{
+	if (size < 0)
+		BUG("Cannot allocate a negative amount: %ld", size);
+	return xmalloc(size);
+}
 #define obstack_chunk_free free
 
 #define U(c) ((unsigned char) (c))