build: use correct CPPFLAGS for out-of-tree build

Generated headers need to be included from the $(builddir).

Also remove unnecessary indirection: AM_CPPFLAGS is used for all
targets that do not have their own foo_CPPFLAGS.

Refs T770, similar to D1571 and D1613.

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D1646

src/Makefile.am

 
 includesfidir = $(includedir)/prestwood
-includes= \
-          -I$(top_srcdir)\
-          -I$(top_srcdir)/interface/ \
-          $(NULL)
+AM_CPPFLAGS = \
+	-I$(top_srcdir) \
+	-I$(top_builddir) \
+	-I$(top_srcdir)/interface \
+	-I$(top_builddir)/interface \
+	$(NULL)
 
 source_c = prestwood-main.c \
 	   prestwood.c
@@ -14,9 +16,6 @@ includesfi_HEADERS = prestwood.h
 
 bin_PROGRAMS = prestwood
 
-prestwood_CPPFLAGS = $(includes) \
-                     $(AM_CPPFLAGS)
-
 prestwood_LDADD = \
                     $(GLIB_LIBS) $(GUPNP_LIBS) $(GSSDP_LIBS)\
 			$(CANTERBURY_LIBS) $(CODE_COVERAGE_LIBS) $(AM_LIBADD) \