Alias Bitu to uintptr_t (resp. Bits and intptr_t)

Unlike the explicitly-sized integer types, it isn’t evident which type
was meant to be used for these two types, so let’s keep them as is until
we decide which ones they should be.

acinclude.m4

@@ -299,12 +299,6 @@ dnl Linux systems for ages now have had stdint.h to define uint8_t, etc.
 AH_BOTTOM([
 #include <cstdint>
 
-#if SIZEOF_INT_P == 4
-  typedef uint32_t Bitu;
-  typedef int32_t Bits;
-#else
-  typedef uint64_t Bitu;
-  typedef int64_t Bits;
-#endif
-
+typedef uintptr_t Bitu;
+typedef intptr_t Bits;
 ])