Replace INT32 with a new internal datatype (JLONG)

These days, INT32 is a commonly-defined datatype in system headers.  We
cannot eliminate the definition of that datatype from jmorecfg.h, since
the INT32 typedef has technically been part of the libjpeg API since
version 5 (1994.)  However, using INT32 internally is risky, because the
inclusion of a particular header (Xmd.h, for instance) could change the
definition of INT32 from long to int on 64-bit platforms and thus change
the internal behavior of libjpeg-turbo in unexpected ways (for instance,
failing to correctly set __INT32_IS_ACTUALLY_LONG to match the INT32
typedef-- perhaps as a result of including the wrong version of
jpeglib.h-- could cause libjpeg-turbo to produce incorrect results.)

The library has always been built in environments in which INT32 is
effectively long (on Windows, long is always 32-bit, so effectively it's
the same as int), so it makes sense to turn INT32 into an explicitly
long datatype.  This ensures that libjpeg-turbo will always behave
consistently, regardless of the headers included at compile time.

Addresses a concern expressed in #26.

jdarith.c

@@ -26,8 +26,8 @@
 typedef struct {
   struct jpeg_entropy_decoder pub; /* public fields */
 
-  INT32 c;       /* C register, base of coding interval + input bit buffer */
-  INT32 a;               /* A register, normalized size of coding interval */
+  JLONG c;       /* C register, base of coding interval + input bit buffer */
+  JLONG a;               /* A register, normalized size of coding interval */
   int ct;     /* bit shift counter, # of bits left in bit buffer part of C */
                                                          /* init: ct = -16 */
                                                          /* run: ct = 0..7 */
@@ -110,7 +110,7 @@ arith_decode (j_decompress_ptr cinfo, unsigned char *st)
 {
   register arith_entropy_ptr e = (arith_entropy_ptr) cinfo->entropy;
   register unsigned char nl, nm;
-  register INT32 qe, temp;
+  register JLONG qe, temp;
   register int sv, data;
 
   /* Renormalization & data input per section D.2.6 */