build: fix link error for `_BitScanForward64` (#28173)

Problem: The usage of `_BitScanForward64` causes linking to fail on some systems.
Solution: Correctly check if it exists using `check_c_source_compiles`.

cmake.config/CMakeLists.txt

@@ -79,6 +79,17 @@ int main(void)
 }
 " HAVE_PWD_FUNCS)
 
+check_c_source_compiles("
+#include <intrin.h>
+
+int main(void)
+{
+  unsigned long index;
+  unsigned char mask = 0x8000;
+  _BitScanForward64(&index, mask);
+  return 0;
+}
+" HAVE_BITSCANFORWARD64)
 
 if(CMAKE_SYSTEM_NAME STREQUAL "SunOS")
   check_c_source_compiles("

cmake.config/config.h.in

@@ -52,3 +52,4 @@
 #cmakedefine HAVE_EXECINFO_BACKTRACE
 #cmakedefine HAVE_BUILTIN_ADD_OVERFLOW
 #cmakedefine HAVE_WIMPLICIT_FALLTHROUGH_FLAG
+#cmakedefine HAVE_BITSCANFORWARD64

src/nvim/math.c

@@ -56,7 +56,7 @@ int xctz(uint64_t x)
   // Use compiler builtin if possible.
 #if defined(__clang__) || (defined(__GNUC__) && (__GNUC__ >= 4))
   return __builtin_ctzll(x);
-#elif defined(_MSC_VER)
+#elif defined(HAVE_BITSCANFORWARD64)
   unsigned long index;
   _BitScanForward64(&index, x);
   return (int)index;