configure: adjust tests to avoid -Werror breakage

These series allow ./configure to run when CFLAGS contains -Werror.

Search for AC_LANG_PROGRAM in *.m4, configure.ac and void-cast all
unused variables. Furthermore, a sin6_scope_id variable is cast void and
two main functions gained an explicit int return value.

While gethostname and inet_ntop in curl-functions.m4 got their
parameters and const-correctness fixed, it was not trivially possible to
fix AC_LANG_FUNC_LINK_TRY which fails with "conflicting types for
built-in function 'strdup'" (for example). This patch does not fix that
remaining error, see also the autoconf manual:

    "It is considered to be severely broken, and in the future will be
    removed in favor of actual function calls (with properly typed
    arguments)."

While at it, update copyright years.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>