Document that -k is available only with libunwind

Without this users might get confused why `man strace` describes
an option that doesn't work.

* strace.1: Note that -k is available only if strace is built
with libunwind.

diff --git a/strace.1 b/strace.1
index 1e69217f35c43c7caa1cf96f3f4e1652cc48a94d..c1f87e9da3c1f260ee1ad81539ad4143d5eb3d4c 100644 (file)
--- a/strace.1
+++ b/strace.1
@@ -269,6 +269,9 @@ Print the instruction pointer at the time of the system call.
 .TP
 .B \-k
 Print the execution stack trace of the traced processes after each system call (experimental).
+This option is available only if
+.B strace
+is built with libunwind.
 .TP
 .B \-q
 Suppress messages about attaching, detaching etc.  This happens