m4: Use x in order to avoid variable 'x' set but not used [-Werror=unused-but-set-variable]

This error could be caused by configure scripts being run with -Werror
-Wall, which would lead to libcurl being detected as unusable.

docs/libcurl/libcurl.m4

@@ -157,6 +157,7 @@ x=CURLOPT_FILE;
 x=CURLOPT_ERRORBUFFER;
 x=CURLOPT_STDERR;
 x=CURLOPT_VERBOSE;
+if (x) ;
 ])],libcurl_cv_lib_curl_usable=yes,libcurl_cv_lib_curl_usable=no)
 
            CPPFLAGS=$_libcurl_save_cppflags