FindThreads: Fix libc check to use proper header for pthread_kill

In commit e9a1ddc594 (FindThreads: Replace the pthread symbol checking
in libc., 2018-11-18, v3.14.0-rc1~292^2) we switched to checking for
`pthread_kill` in libc but did not update the symbol check's header file
to match.  Add `signal.h` to get `pthread_kill`.  Keep `pthread.h`
anyway since the purpose of the check is to verify that the pthread API
works.

Fixes: #18984

1 files changed, 4 insertions, 2 deletions

diff --git a/Modules/FindThreads.cmake b/Modules/FindThreads.cmake
index 5d894c8..a2f3fa3 100644
--- a/Modules/FindThreads.cmake
+++ b/Modules/FindThreads.cmake
@@ -125,8 +125,10 @@ if(CMAKE_HAVE_PTHREAD_H)
     # libc(libasan). However, it doesn't have the full pthread implementation.
     # So, we can't assume that we have the pthread implementation in libc
     # using the pthread_create() checking here. Then, we turn to check the
-    # pthread_kill() symbol instead.
-    CHECK_SYMBOL_EXISTS(pthread_kill pthread.h CMAKE_HAVE_LIBC_PTHREAD_KILL)
+    # pthread_kill() symbol instead.  It is in the "signal.h" header, but
+    # the purpose of this check is to verify that the pthread API works with
+    # pure libc, so verify that "pthread.h" can be included too.
+    CHECK_SYMBOL_EXISTS(pthread_kill "signal.h;pthread.h" CMAKE_HAVE_LIBC_PTHREAD_KILL)
     if(CMAKE_HAVE_LIBC_PTHREAD_KILL)
       set(CMAKE_THREAD_LIBS_INIT "")
       set(CMAKE_HAVE_THREADS_LIBRARY 1)