SIGSTKSZ is -1 in glibc 2.34

[heiner]

glibc 2.34 sets the SIGSTKSZ constant to -1 when _GNU_SOURCE or _SC_SIGSTKSZ_SOURCE are defined: https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=6c57d320484988e87e446e2e60ce42816bf51d53

This caused a number of issues in various projects, e.g., vim/vim#7895. It also breaks the default stack size logic when using create_fcontext_stack(0). The suggested alternative is apparently sysconf(_SC_SIGSTKSZ), which has the downside of being GNU/Linux specific.