Fix check for using computed goto flag for re2c

[petk]

AC_LANG_SOURCE should be used instead of the AC_LANG_PROG so the compilation check can succeed when the computed goto extension is available to optimize conditional jumps. This one was missed when we were updating the Autoconf AC_TRY_COMPILE macros:

Steps to reproduce (Linux with GCC):

./buildconf
./configure --enable-re2c-cgoto

The -g flag for re2c isn't enabled (but could be):

...
checking whether to enable computed goto gcc extension with re2c... yes
checking whether re2c -g works... no
...

Update: This needs to be further fixed. This isn't working either.

[petk]

I've fixed this now properly. Otherwise, now when using the ./configure --enable-re2c-cgoto, the -g flag is appended as it should be but then there are compilation uninitialized warnings probably due to some re2c bugs (I've tested this on re2c 3.0 and 3.1). Maybe this should go into PHP-8.3 only, since it can change the way software is built out there and to have as little worries if something won't work somewhere:

Zend/zend_language_scanner.c: In function ‘lex_scan’:
Zend/zend_language_scanner.c:1997:39: warning: ‘yych’ may be used uninitialized [-Wmaybe-uninitialized]
 1997 |                         goto *yytarget[yych];
      |                               ~~~~~~~~^~~~~~
Zend/zend_language_scanner.c:1354:17: note: ‘yych’ was declared here
 1354 |         YYCTYPE yych;
      |                 ^~~~

Code compiles and tests succeed properly otherwise.

[petk]

Fix updated and adjusted. Targeting master branch (PHP-8.4-dev).