configure.ac: Avoid that automake complains that AM_PROG_AR is missing

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
configure.ac: Enable the subdir-objects option

Avoid that automake >= 1.14 warns that some source files are in a
subdirectory but that the option subdir-objects is disabled. See also
http://www.gnu.org/software/automake/manual/html_node/List-of-Automake-options.html#index-Options_002c-subdir_002dobjects

Signed-off-by: Bart Van Assche <bvanassche@acm.org>