`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 #14097Closes#14122
b05dc7eb35