lib: feature deprecation warnings in gcc >= 4.3

Add a deprecated attribute to functions and enum values that should not
be used anymore.
This uses a gcc 4.3 dialect, thus is only available for this version of
gcc and newer. Note that the _Pragma() keyword is introduced by C99, but
is available as part of the gcc dialect even when compiling in C89 mode.

It is still possible to disable deprecation at a calling module compile
time by defining CURL_DISABLE_DEPRECATION.

Gcc type checking macros are made aware of possible deprecations.

Some testing support Perl programs are adapted to the extended
declaration syntax.

Several test and unit test C programs intentionally use deprecated
functions/options and are annotated to not generate a warning.

New test 1222 checks the deprecation status in doc and header files.

Closes #9667

tests/unit/unit1308.c

@@ -21,6 +21,7 @@
  * SPDX-License-Identifier: curl
  *
  ***************************************************************************/
+#define CURL_DISABLE_DEPRECATION  /* Testing the form api */
 #include "curlcheck.h"
 
 #include <curl/curl.h>