build: improve builtin_clz* detection

The way we currently test with AC_CHECK_DECLS do not work with Clang:
```bash
configure:21492: clang++-10 -std=c++11 -c -g -O2  -DHAVE_BUILD_INFO -D__STDC_FORMAT_MACROS conftest.cpp >&5
conftest.cpp:100:10: error: builtin functions must be directly called
  (void) __builtin_clz;
         ^
1 error generated.
```

This also removes the __builtin_clz() check, as we don't actually use
it anywhere, and it's trvial to re-add detection if we do start using
it at some point.

src/crypto/common.h

@@ -82,12 +82,12 @@ void static inline WriteBE64(unsigned char* ptr, uint64_t x)
 /** Return the smallest number n such that (x >> n) == 0 (or 64 if the highest bit in x is set. */
 uint64_t static inline CountBits(uint64_t x)
 {
-#if HAVE_DECL___BUILTIN_CLZL
+#if HAVE_BUILTIN_CLZL
     if (sizeof(unsigned long) >= sizeof(uint64_t)) {
         return x ? 8 * sizeof(unsigned long) - __builtin_clzl(x) : 0;
     }
 #endif
-#if HAVE_DECL___BUILTIN_CLZLL
+#if HAVE_BUILTIN_CLZLL
     if (sizeof(unsigned long long) >= sizeof(uint64_t)) {
         return x ? 8 * sizeof(unsigned long long) - __builtin_clzll(x) : 0;
     }