Improve libunwind detection.

http://code.google.com/p/google-glog/issues/detail?id=42


git-svn-id: https://google-glog.googlecode.com/svn/trunk@84 eb4d4688-79bd-11dd-afb4-1d65580434c0

configure

@@ -21133,7 +21133,9 @@ if test `eval echo '${'$as_ac_Header'}'` = yes; then
   cat >>confdefs.h <<_ACEOF
 #define `echo "HAVE_$ac_header" | $as_tr_cpp` 1
 _ACEOF
-
+ ac_cv_have_libunwind_h=1
+else
+  ac_cv_have_libunwind_h=0
 fi
 
 done

configure.ac

@@ -36,7 +36,7 @@ AC_CHECK_HEADERS(sys/syscall.h)
 # For backtrace with glibc.
 AC_CHECK_HEADERS(execinfo.h)
 # For backtrace with libunwind.
-AC_CHECK_HEADERS(libunwind.h)
+AC_CHECK_HEADERS(libunwind.h, ac_cv_have_libunwind_h=1, ac_cv_have_libunwind_h=0)
 AC_CHECK_HEADERS(ucontext.h)
 AC_CHECK_HEADERS(sys/utsname.h)
 AC_CHECK_HEADERS(pwd.h)