Fix -Wundef warnings for patterns `#if HAVE`

* See [Feature #17752]
* Using this to detect them:
  git grep -P 'if\s+HAVE' | grep -Pv 'HAVE_LONG_LONG|/ChangeLog|HAVE_TYPEOF'

1 files changed, 1 insertions, 1 deletions

diff --git a/signal.c b/signal.c
index 2ebd1196fa..46ac6c26cc 100644
--- a/signal.c
+++ b/signal.c
@@ -29,7 +29,7 @@
 # include <ucontext.h>
 #endif
 
-#if HAVE_PTHREAD_H
+#ifdef HAVE_PTHREAD_H
 # include <pthread.h>
 #endif