build: use __SIZEOF_INT128__ for checking __int128 availability

We already use this in the blockfilter code, so not sure we need to maintain two
different ways of testing for the same functionality. Consolidate on testing
for __SIZEOF_INT128__, which we already use, is supported by the compilers we
care about, and is also used by libsecp256k1.

Author: fanquake

configure.ac

@@ -837,9 +837,6 @@ if test "$use_lcov_branch" != "no"; then
   AC_SUBST(LCOV_OPTS, "$LCOV_OPTS --rc lcov_branch_coverage=1")
 fi
 
-dnl Check for __int128
-AC_CHECK_TYPES([__int128])
-
 dnl Check for endianness
 AC_C_BIGENDIAN
 

src/crypto/muhash.h

@@ -24,7 +24,7 @@ class Num3072
 public:
     static constexpr size_t BYTE_SIZE = 384;
 
-#ifdef HAVE___INT128
+#ifdef __SIZEOF_INT128__
     typedef unsigned __int128 double_limb_t;
     typedef uint64_t limb_t;
     static constexpr int LIMBS = 48;