configure: Properly handle case where libunwind.h is not available.

configure: Properly handle case where libunwind.h is not available.

It is possible that user install libunwind but not libunwind-devel,
and it will run into a compilation error.  So we need to check the
existence of the library and the header file.

Fixes: e2ed6fbeb18c ("fatal-signal: Catch SIGSEGV and print backtrace.")
Suggested-by: Ben Pfaff <blp@ovn.org>
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: William Tu <u9012063@gmail.com>