Add libdwfl based implementation

[kanje]

This commit adds an implementation based on libdwfl from elfutils.

Implements #176

[kanje]

@apolukhin, please have a look at this draft. Note, I was using CMake and b2 support is still missing. I would appreciate some help here as I know nothing about how to write Jamfiles.

[apolukhin]

Tests are missing. You will need to write some b2 scripts: follow this sample

stacktrace/build/Jamfile.v2

Lines 80 to 93 in 344f6ba

	lib boost_stacktrace_backtrace
	: # sources
	../src/backtrace.cpp
	: # requirements
	<warnings>all
	<target-os>linux:<library>dl
	<library>backtrace
	<link>shared:<define>BOOST_STACKTRACE_DYN_LINK=1
	[ check-target-builds libbacktrace : : <build>no ]
	: # default build
	: # usage-requirements
	#<link>shared:<define>BOOST_STACKTRACE_DYN_LINK=1
	<define>BOOST_STACKTRACE_NO_LIB=1
	;

and add code to search for libdwfl

stacktrace/build/Jamfile.v2

Lines 34 to 40 in 344f6ba

	local LIBBACKTRACE_PATH = [ modules.peek : LIBBACKTRACE_PATH ] ;
	lib backtrace
	:
	: <search>$(LIBBACKTRACE_PATH)/lib <link>static
	:
	: <include>$(LIBBACKTRACE_PATH)/include
	;

After that adjust https://github.com/boostorg/stacktrace/blob/develop/test/Jamfile.v2 by adding each of the tests with libdwfl