configure: support libunwind of LLVM

The libunwind headers of LLVM are located in the subdirectory
/usr/include/libunwind. Search that subdirectory when the default
header test fails. Also extend the include path due to the transitive
include of `<__libunwind_config.h>`.

Closes: #894

configure.ac

@@ -419,13 +419,23 @@ case "$enable_unwind" in
  AC_CHECK_LIB([lzma], [lzma_index_buffer_decode])
  fi
  AC_CHECK_LIB([unwind], [backtrace], [], [enable_unwind=no])
- AC_CHECK_HEADERS([libunwind.h], [], [enable_unwind=no])
+ AC_CHECK_HEADERS([libunwind.h], [], [
+ old_CFLAGS="$CFLAGS"
+ CFLAGS="$CFLAGS -I/usr/include/libunwind"
+ AC_CHECK_HEADERS([libunwind/libunwind.h], [], [
+ enable_unwind=no
+ CFLAGS="$old_CFLAGS"
+ ])
+ ])
  ;;
  no)
  ;;
  yes)
  AC_CHECK_LIB([unwind], [backtrace], [], [AC_MSG_ERROR([can not find required library libunwind])])
- AC_CHECK_HEADERS([libunwind.h], [], [AC_MSG_ERROR([can not find require header file libunwind.h])])
+ AC_CHECK_HEADERS([libunwind.h], [], [
+ CFLAGS="$CFLAGS -I/usr/include/libunwind"
+ AC_CHECK_HEADERS([libunwind/libunwind.h], [], [AC_MSG_ERROR([can not find required header file libunwind.h])])
+ ])
  ;;
  *)
  AC_MSG_ERROR([bad value '$enable_unwind' for --enable-unwind])