Use -lpthread on OpenBSD

cmake/sdlchecks.cmake

@@ -905,7 +905,7 @@ macro(CheckPTHREAD)
       set(PTHREAD_LDFLAGS "-lpthread")
     elseif(OPENBSD)
       set(PTHREAD_CFLAGS "-D_REENTRANT")
-      set(PTHREAD_LDFLAGS "-pthread")
+      set(PTHREAD_LDFLAGS "-lpthread")
     elseif(SOLARIS)
       set(PTHREAD_CFLAGS "-D_REENTRANT")
       set(PTHREAD_LDFLAGS "-pthread -lposix4")

configure

@@ -26778,7 +26778,7 @@ fi
             ;;
         *-*-openbsd*)
             pthread_cflags="-D_REENTRANT"
-            pthread_lib="-pthread"
+            pthread_lib="-lpthread"
             ;;
         *-*-solaris2.9)
             # From Solaris 9+, posix4's preferred name is rt.

configure.ac

@@ -3063,7 +3063,7 @@ dnl This is used on Linux for glibc binary compatibility (Doh!)
             ;;
         *-*-openbsd*)
             pthread_cflags="-D_REENTRANT"
-            pthread_lib="-pthread"
+            pthread_lib="-lpthread"
             ;;
         *-*-solaris2.9)
             # From Solaris 9+, posix4's preferred name is rt.