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💯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.

build_msvc/bitcoin_config.h

@@ -137,18 +137,6 @@
    don't. */
 #define HAVE_DECL_STRNLEN 1
 
-/* Define to 1 if you have the declaration of `__builtin_clz', and to 0 if you
-   don't. */
-//#define HAVE_DECL___BUILTIN_CLZ 1
-
-/* Define to 1 if you have the declaration of `__builtin_clzl', and to 0 if
-   you don't. */
-//#define HAVE_DECL___BUILTIN_CLZL 1
-
-/* Define to 1 if you have the declaration of `__builtin_clzll', and to 0 if
-   you don't. */
-//#define HAVE_DECL___BUILTIN_CLZLL 1
-
 /* Define to 1 if you have the <dlfcn.h> header file. */
 /* #undef HAVE_DLFCN_H */
 

configure.ac

@@ -849,7 +849,21 @@ AC_CHECK_DECLS([bswap_16, bswap_32, bswap_64],,,
                  #include <byteswap.h>
                  #endif])
 
-AC_CHECK_DECLS([__builtin_clz, __builtin_clzl, __builtin_clzll])
+AC_MSG_CHECKING(for __builtin_clzl)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[ ]], [[
+ (void) __builtin_clzl(0);
+  ]])],
+ [ AC_MSG_RESULT(yes); AC_DEFINE(HAVE_BUILTIN_CLZL, 1, [Define this symbol if you have __builtin_clzl])],
+ [ AC_MSG_RESULT(no)]
+)
+
+AC_MSG_CHECKING(for __builtin_clzll)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[ ]], [[
+  (void) __builtin_clzll(0);
+  ]])],
+ [ AC_MSG_RESULT(yes); AC_DEFINE(HAVE_BUILTIN_CLZLL, 1, [Define this symbol if you have __builtin_clzll])],
+ [ AC_MSG_RESULT(no)]
+)
 
 dnl Check for malloc_info (for memory statistics information in getmemoryinfo)
 AC_MSG_CHECKING(for getmemoryinfo)

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;
     }