In imath.h, replace stdint.h usage with c.h equivalents.

As things stood, buildfarm member dory failed.  MSVC versions lacking
stdint.h are unusable for building PostgreSQL, but pg_config.h.win32
doesn't know that.  Even so, we support other systems lacking stdint.h,
including buildfarm member gaur.  Per a suggestion from Tom Lane.

Discussion: https://postgr.es/m/9598.1550353336@sss.pgh.pa.us

contrib/pgcrypto/imath.c

@@ -16,6 +16,7 @@
  *    - #undef MIN, #undef MAX before defining them
  *    - remove includes covered by c.h
  *    - rename DEBUG to IMATH_DEBUG
+ *    - replace stdint.h usage with c.h equivalents
  *
  * 2. Download a newer imath.c and imath.h.  Transform them like in step 1.
  *    Apply to these files the diff you saved in step 1.  Look for new lines

contrib/pgcrypto/imath.h

@@ -36,17 +36,17 @@ typedef long mp_small;			/* must be a signed type */
 typedef unsigned long mp_usmall;	/* must be an unsigned type */
 
 
-/* Build with words as uint64_t by default. */
+/* Build with words as uint64 by default. */
 #ifdef USE_32BIT_WORDS
-typedef uint16_t mp_digit;
-typedef uint32_t mp_word;
-#define MP_DIGIT_MAX  (UINT16_MAX * 1UL)
-#define MP_WORD_MAX   (UINT32_MAX * 1UL)
+typedef uint16 mp_digit;
+typedef uint32 mp_word;
+#define MP_DIGIT_MAX  (PG_UINT16_MAX * 1UL)
+#define MP_WORD_MAX   (PG_UINT32_MAX * 1UL)
 #else
-typedef uint32_t mp_digit;
-typedef uint64_t mp_word;
-#define MP_DIGIT_MAX  (UINT32_MAX * UINT64_C(1))
-#define MP_WORD_MAX   (UINT64_MAX)
+typedef uint32 mp_digit;
+typedef uint64 mp_word;
+#define MP_DIGIT_MAX  (PG_UINT32_MAX * UINT64CONST(1))
+#define MP_WORD_MAX   (PG_UINT64_MAX)
 #endif
 
 typedef struct