Find provider for backtrace(3).

Checks if OS supports backtrace(3) via either libc or custom library. This module defines the following variables:

Backtrace_HEADER       - The header file needed for backtrace(3). Cached.
                         Could be forcibly set by user.
Backtrace_INCLUDE_DIRS - The include directories needed to use backtrace(3) header.
Backtrace_LIBRARIES    - The libraries (linker flags) needed to use backtrace(3), if any.
Backtrace_FOUND        - Is set if and only if backtrace(3) support detected.

The following cache variables are also available to set or use:

Backtrace_LIBRARY     - The external library providing backtrace, if any.
Backtrace_INCLUDE_DIR - The directory holding the backtrace(3) header.

Typical usage is to generate of header file using configure_file() with the contents like the following:

#cmakedefine01 Backtrace_FOUND
#if Backtrace_FOUND
# include <${Backtrace_HEADER}>

And then reference that generated header file in actual source.