build: fix Codacy/CppCheck warnings

- remove unused variables
- declare conditionally used variables conditionally
- suppress unused variable warnings in the CMake tests
- remove dead variable stores
- consistently use WIN32 macro to detect Windows

Closes https://github.com/curl/curl/pull/3739

src/tool_cb_prg.c

@@ -221,7 +221,7 @@ void progressbarinit(struct ProgressData *bar,
     struct winsize ts;
     if(!ioctl(STDIN_FILENO, TIOCGWINSZ, &ts))
       cols = ts.ws_col;
-#elif defined(_WIN32)
+#elif defined(WIN32)
     {
       HANDLE  stderr_hnd = GetStdHandle(STD_ERROR_HANDLE);
       CONSOLE_SCREEN_BUFFER_INFO console_info;

src/tool_cb_wrt.c

@@ -159,7 +159,7 @@ size_t tool_write_cb(char *buffer, size_t sz, size_t nmemb, void *userdata)
     }
   }
 
-#ifdef _WIN32
+#ifdef WIN32
   fhnd = _get_osfhandle(fileno(outs->stream));
   if(isatty(fileno(outs->stream)) &&
      GetConsoleScreenBufferInfo((HANDLE)fhnd, &console_info)) {

src/tool_doswin.c

@@ -599,7 +599,6 @@ SANITIZEcode rename_if_reserved_dos_device_name(char **const sanitized,
       }
       memmove(base + 1, base, blen + 1);
       base[0] = '_';
-      ++blen;
     }
   }
 #endif

src/tool_main.c

@@ -237,7 +237,7 @@ static void main_free(struct GlobalConfig *config)
   config->last = NULL;
 }
 
-#ifdef _WIN32
+#ifdef WIN32
 /* TerminalSettings for Windows */
 static struct TerminalSettings {
   HANDLE hStdOut;
@@ -275,7 +275,7 @@ static void configure_terminal(void)
 
 static void restore_terminal(void)
 {
-#ifdef _WIN32
+#ifdef WIN32
   /* Restore Console output mode and codepage to whatever they were
    * when Curl started */
   SetConsoleMode(TerminalSettings.hStdOut, TerminalSettings.dwOutputMode);

src/tool_metalink.c

@@ -73,7 +73,7 @@
    and later. If you're building for an older cat, well, sorry. */
 #  define COMMON_DIGEST_FOR_OPENSSL
 #  include <CommonCrypto/CommonDigest.h>
-#elif defined(_WIN32)
+#elif defined(WIN32)
 /* For Windows: If no other crypto library is provided, we fallback
    to the hash functions provided within the Microsoft Windows CryptoAPI */
 #  include <wincrypt.h>
@@ -380,7 +380,7 @@ static void SHA256_Final(unsigned char digest[32], SHA256_CTX *ctx)
   sha256_finish(ctx, digest);
 }
 
-#elif defined(_WIN32)
+#elif defined(WIN32)
 
 static void win32_crypto_final(struct win32_crypto_hash *ctx,
                                unsigned char *digest,