sectransp: fix `HAVE_BUILTIN_AVAILABLE` checks to not emit warnings

sectransp: fix `HAVE_BUILTIN_AVAILABLE` checks to not emit warnings

`HAVE_BUILTIN_AVAILABLE` is a curl macro set via autotools and cmake.
Like other `HAVE_`s it signals availability if defined.

SecureTransport code was specifically looking for the value 1, which
triggered compiler warnings when the feature was not present.

Replace the existing workaround of locally suppressing the compiler
warning with using `defined()`.

autotools:
```
767 | #if (CURL_BUILD_MAC_10_13 || CURL_BUILD_IOS_11) && HAVE_BUILTIN_AVAILABLE == 1
      |                                                    ^~~~~~~~~~~~~~~~~~~~~~
../../lib/vtls/sectransp.c: In function 'sectransp_connect_step1':
../../lib/vtls/sectransp.c:1140:52: error: "HAVE_BUILTIN_AVAILABLE" is not defined, evaluates to 0 [-Werror=undef]
 1140 | #if (CURL_BUILD_MAC_10_13 || CURL_BUILD_IOS_11) && HAVE_BUILTIN_AVAILABLE == 1
      |                                                    ^~~~~~~~~~~~~~~~~~~~~~
../../lib/vtls/sectransp.c:1240:52: error: "HAVE_BUILTIN_AVAILABLE" is not defined, evaluates to 0 [-Werror=undef]
 1240 | #if (CURL_BUILD_MAC_10_13 || CURL_BUILD_IOS_11) && HAVE_BUILTIN_AVAILABLE == 1
      |                                                    ^~~~~~~~~~~~~~~~~~~~~~
../../lib/vtls/sectransp.c: In function 'sectransp_connect_step2':
```
Ref: https://github.com/curl/curl/actions/runs/9815428701/job/27104448045#step:6:499

cmake gcc:
```
 1140 | #if (CURL_BUILD_MAC_10_13 || CURL_BUILD_IOS_11) && HAVE_BUILTIN_AVAILABLE == 1
      |                                                    ^~~~~~~~~~~~~~~~~~~~~~
/Users/runner/work/curl/curl/lib/vtls/sectransp.c:1240:52: error: "HAVE_BUILTIN_AVAILABLE" is not defined, evaluates to 0 [-Werror=undef]
 1240 | #if (CURL_BUILD_MAC_10_13 || CURL_BUILD_IOS_11) && HAVE_BUILTIN_AVAILABLE == 1
      |                                                    ^~~~~~~~~~~~~~~~~~~~~~
/Users/runner/work/curl/curl/lib/vtls/sectransp.c: In function 'sectransp_connect_step2':
/Users/runner/work/curl/curl/lib/vtls/sectransp.c:2231:51: error: "HAVE_BUILTIN_AVAILABLE" is not defined, evaluates to 0 [-Werror=undef]
 2231 | #if(CURL_BUILD_MAC_10_13 || CURL_BUILD_IOS_11) && HAVE_BUILTIN_AVAILABLE == 1
      |                                                   ^~~~~~~~~~~~~~~~~~~~~~
```
Ref: https://github.com/curl/curl/actions/runs/9815428701/job/27104445425#step:8:355

Cherry-picked from #14097
Closes #14122