The latest glibc has redefined SIGSTKSZ as 'sysconf (_SC_SIGSTKSZ)' and
this breaks a static char[] definition.
Hardcoding TESTSIGSTKSZ to 16384 in the test. This fixes:
sigaltstack.c:17:13: error: variably modified 'stack_thread' at file scope
17 | static char stack_thread[SIGSTKSZ + TEST_MSG_BUFFER_SIZE] __stack_aligned__;
| ^~~~~~~~~~~~
sigaltstack.c:18:13: error: variably modified 'stack_main' at file scope
18 | static char stack_main[SIGSTKSZ + TEST_MSG_BUFFER_SIZE] __stack_aligned__;
| ^~~~~~~~~~
Signed-off-by: Adrian Reber <areber@redhat.com>