build-sys: Minor makefile tweaks

I'm introducing a new binary in a later patch, and it makes
sense to move more things to be common into the common section.

Also I noticed we were missing an inclusion of common `$(AM_LDFLAGS)`, though
AFAIK this doesn't break anything right now.

Closes: #636
Approved by: jlebon

Makefile-decls.am

@@ -30,6 +30,7 @@ sbin_PROGRAMS =
 bin_SCRIPTS =
 lib_LTLIBRARIES =
 libexec_PROGRAMS =
+pkglibexec_PROGRAMS =
 pkglibexec_SCRIPTS =
 noinst_LTLIBRARIES =
 noinst_PROGRAMS =

Makefile-ostree.am

@@ -102,12 +102,14 @@ src/ostree/parse-datetime.c: src/ostree/parse-datetime.y Makefile
 EXTRA_DIST += src/ostree/parse-datetime.y
 CLEANFILES += src/ostree/parse-datetime.c
 
-ostree_bin_shared_cflags = $(AM_CFLAGS) -I$(srcdir)/src/libotutil -I$(srcdir)/src/libostree -I$(srcdir)/src/ostree \
-	$(NULL)
-ostree_bin_shared_ldadd = libglnx.la libbsdiff.la libotutil.la libostree-kernel-args.la libostree-1.la
+ostree_bin_shared_cflags = $(AM_CFLAGS) -I$(srcdir)/src/libotutil -I$(srcdir)/src/libostree \
+	-I$(srcdir)/src/ostree -I$(srcdir)/libglnx $(OT_INTERNAL_GIO_UNIX_CFLAGS)
+ostree_bin_shared_ldadd = $(AM_LDFLAGS) libglnx.la libotutil.la libostree-1.la \
+	$(OT_INTERNAL_GIO_UNIX_LIBS)
+
+ostree_CFLAGS = $(ostree_bin_shared_cflags)
+ostree_LDADD = $(ostree_bin_shared_ldadd) libbsdiff.la libostree-kernel-args.la $(LIBSYSTEMD_LIBS)
 
-ostree_CFLAGS = $(ostree_bin_shared_cflags) $(OT_INTERNAL_GIO_UNIX_CFLAGS) -I$(srcdir)/libglnx
-ostree_LDADD = $(ostree_bin_shared_ldadd) $(OT_INTERNAL_GIO_UNIX_LIBS) $(LIBSYSTEMD_LIBS)
 
 if USE_LIBSOUP
 ostree_SOURCES += \