remove printf from signal handlers
use AC_SEARCH_LIB in configure.ac (shorter code)
replace '-lm -lcrypto -lz' by  @LIBS@ in Makefile.in and AC_CHECK_LIB macros in configure.ac
replace NO_BACKTRACE by HAVE_EXECINFO_H
Merge pull request #56 from antma/master

remove printf from signal handlers and reduce configure.ac