fix typo in helper_thread.c and run block_signals(void) function in linux only.

[shailevi23]

fix typo in helper_thread.c and run block_signals(void) function in linux only.

Compiling the code with the typo fix of "#ifdef CONFIG_PTHREAD_SIGMASK" will show that ret is unknown, hence, we are adding the line "int ret;". This fix of typo will fix some other bugs I've encountered.

Also, sigmask works in linux environment, so we will if defined it to linux only.

Signed-off-by: Shai Levy shailevy23@gmail.com.

[vincentkfu]

Your new patch will prevent the code from compiling on platforms like BSD and Solaris.

Please follow the suggestion I made in your earlier PR. Evidence suggests that the Windows msys2 build failed because on that platform pthread_sigmask is defined as a noop which triggered the unused variable warning for sigmask.

The configure script detects the existence of pthread_sigmask by trying to compile this program:

#include <stddef.h> /* NULL */
#include <signal.h> /* pthread_sigmask() */
int main(void)
{
  return pthread_sigmask(0, NULL, NULL);
}

Change this program to also trigger the same warning so that compilation also fails. This will make CONFIG_PTHREAD_SIGMASK undefined for the Windows msys2 build and then your original fix will pass all of our testing.