build-sys: use AC_COMPILE_IFELSE

The AC_TRY_COMPILE is obsolete.

Addresses: http://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/html_node/Obsolete-Macros.html#index-AC_005fTRY_005fCOMPILE-2203
Signed-off-by: Sami Kerola <kerolasa@iki.fi>
build-sys: use AC_USE_SYSTEM_EXTENSIONS

The AC_GNU_SOURCE is obsolete.

Addresses: http://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/html_node/Obsolete-Macros.html#index-AC_005fGNU_005fSOURCE-2058
Signed-off-by: Sami Kerola <kerolasa@iki.fi>
build-sys: add package url to AC_INIT

See last line from './configure --help'.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>