configure.ac: fix automake 1.14 "option 'subdir-objects' is disabled" warning

Fix the following warning when using automake-1.14:

automake-1.14: warning: possible forward-incompatibility.
automake-1.14: At least a source file is in a subdirectory, but the
'subdir-objects'
automake-1.14: automake option hasn't been enabled.  For now, the
corresponding output
automake-1.14: object file(s) will be placed in the top-level
directory.  However,
automake-1.14: this behaviour will change in future Automake versions: they will
automake-1.14: unconditionally cause object files to be placed in the
same subdirectory
automake-1.14: of the corresponding sources.
automake-1.14: You are advised to start using 'subdir-objects' option
throughout your
automake-1.14: project, to avoid future incompatibilities.
src/Makefile.am:130: warning: source file 'tables/???.c' is in a subdirectory,
src/Makefile.am:130: but option 'subdir-objects' is disabled
src/Makefile.am:130: warning: source file 'tables/atsc_???.c' is in a
subdirectory,
src/Makefile.am:130: but option 'subdir-objects' is disabled
src/Makefile.am:77: warning: source file 'descriptors/dr_??.c' is in a
subdirectory,
src/Makefile.am:77: but option 'subdir-objects' is disabled
Signed-off-by: Michael Ira Krufky <mkrufky@linuxtv.org>
Signed-off-by: Jean-Paul Saman <jpsaman@videolan.org>

configure.ac

@@ -4,7 +4,7 @@ AC_PREREQ(2.50)
 AC_CONFIG_AUX_DIR(.auto)
 AC_CANONICAL_SYSTEM
 
-AM_INIT_AUTOMAKE
+AM_INIT_AUTOMAKE([subdir-objects])
 AC_CONFIG_HEADERS(config.h)
 AC_CONFIG_MACRO_DIR([m4])