libbacktrace: load Windows modules

Patch from Björn Schäpers <bjoern@hazardy.de>.

	* configure.ac: Checked for tlhelp32.h
	* pecoff.c: Include <tlhelp32.h> if available.
	(backtrace_initialize): Use tlhelp32 api for a snapshot to
	detect loaded modules.
	(coff_add): New argument for the module handle of the file,
	to get the base address.
	* configure, config.h.in: Regenerate.