Build: reference docs: Build when srcdir != builddir

docs/libglom_reference/Makefile.am

@@ -18,7 +18,8 @@
 include $(top_srcdir)/glom/libglom/filelist.am
 
 book_name  = libglom-$(GLOM_ABI_VERSION)
-doc_input  = $(addprefix $(top_srcdir)/,$(libglom_headers))
+doc_input  = $(addprefix $(top_srcdir)/,$(libglom_headers)) \
+	     $(addprefix $(top_builddir)/,$(libglom_headers_generated))
 doc_outdir = .
 
 include $(top_srcdir)/doc-reference.am

glom/libglom/filelist.am

@@ -21,7 +21,6 @@ libglom_toplevel_headers =				\
 	glom/libglom/appstate.h				\
 	glom/libglom/algorithms_utils.h \
 	glom/libglom/init.h				\
-	glom/libglom/libglom_config.h			\
 	glom/libglom/sharedptr.h                        \
 	glom/libglom/standard_table_prefs_fields.h	\
 	glom/libglom/utils.h				\
@@ -32,6 +31,10 @@ libglom_toplevel_headers =				\
 	glom/libglom/report_builder.h \
 	glom/libglom/translations_po.h
 
+# These will be in the build directory, not the source directory.
+libglom_toplevel_headers_generated  = \
+	glom/libglom/libglom_config.h
+
 libglom_data_structure_headers =				\
 	glom/libglom/data_structure/database_title.h		\
 	glom/libglom/data_structure/choicevalue.h		\
@@ -98,6 +101,10 @@ libglom_headers =				\
 	$(libglom_d_bakery_headers)		\
 	$(libglom_d_b_view_headers)
 
+# These will be in the build directory, not the source directory.
+libglom_headers_generated = \
+	$(libglom_toplevel_headers_generated)
+
 # Private source and header files:
 
 libglom_sources =							\