added a lot more options

Makefile:
    changed default CFLAGS to use -std=gnu11

configure.ac:
    addition of AC_CHECK_HEADER([assert.h])

btcapi.h:
    small stylistic changes

cmdlineutils.h:
    defined macro 'noreturn' to C11's _Noreturn
    changed exit functions (help, version) to _Noreturn functions
    set all arguments arguments to const char *const

cmdlineutils.c:
    added new options to help() - -b, -p, -s

main.c:
    inclusion of assert.h
    addition of -b, -p, -s arguments
    various optimisations