[PATCH] use libunwind only when both header and lib available

[Rob Latham]

libunwind has runtime libraries and compile-time heaers.  Some systems
might provide libunwind without the corresponding -dev package.  Be more
careful about when to choose libunwind.
---
 src/mpl/src/bt/mpl_bt.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/mpl/src/bt/mpl_bt.c b/src/mpl/src/bt/mpl_bt.c
index 5abd999d..3102a7a3 100644
--- a/src/mpl/src/bt/mpl_bt.c
+++ b/src/mpl/src/bt/mpl_bt.c
@@ -44,7 +44,11 @@ static inline void backtrace_libback(FILE *output)
     btstate = backtrace_create_state(NULL, 1, NULL, NULL);
     backtrace_print(btstate, 0, output);
 }
-#elif defined MPL_HAVE_LIBUNWIND
+/* we need not only the symbols but the header file too (for the cursor and
+ * context), so tighten up when we take the libunwind path.  Thanks
+ * Siegmar.Gross at informatik.hs-fulda.de for the bug report about systems with
+ * libunwind libraries but no libunwind development headers */
+#if defined MPL_HAVE_LIBUNWIND && defined(MPL_HAVE_LIBUNWIND_H)
 static inline void backtrace_libunwind(FILE *output)
 {
     unw_cursor_t cursor;
-- 
2.5.0


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