broadway backend: allow building on OpenBSD

On OpenBSD shm_open(3) is part of libc and there is not librt, so add an
according check.
Check if we have posix_fallocate before using it.

https://bugzilla.gnome.org/show_bug.cgi?id=728272

configure.ac

@@ -397,6 +397,11 @@ else
 fi
 AC_SUBST(DISABLE_ON_QUARTZ)
 
+AC_CHECK_LIB([rt], [shm_open], [SHM_LIBS="-lrt"], [SHM_LIBS=""])
+AC_SUBST(SHM_LIBS)
+
+AC_CHECK_FUNCS(posix_fallocate)
+
 if test "x$enable_broadway_backend" = xyes; then
   GDK_BACKENDS="$GDK_BACKENDS broadway"
   cairo_backends="$cairo_backends cairo"

gdk/broadway/Makefile.am

@@ -1,5 +1,6 @@
 ## Process this file with automake to produce Makefile.in
 include $(top_srcdir)/Makefile.decl
+NULL =
 
 libgdkincludedir = $(includedir)/gtk-3.0/gdk
 libgdkbroadwayincludedir = $(includedir)/gtk-3.0/gdk/broadway
@@ -75,7 +76,8 @@ libgdk_broadway_la_SOURCES =		\
 
 if OS_UNIX
 libgdk_broadway_la_LIBADD = \
-	-lrt
+	$(SHM_LIBS)         \
+	$(NULL)
 endif
 
 broadwayd_SOURCES = \
@@ -91,7 +93,7 @@ broadwayd_SOURCES = \
 if OS_WIN32
 broadwayd_LDADD = $(GDK_DEP_LIBS) -lws2_32
 else
-broadwayd_LDADD = $(GDK_DEP_LIBS) -lrt
+broadwayd_LDADD = $(GDK_DEP_LIBS) @SHM_LIBS@
 endif
 
 MAINTAINERCLEANFILES = $(broadway_built_sources)

gdk/broadway/gdkbroadway-server.c

@@ -540,12 +540,14 @@ map_named_shm (char *name, gsize size)
   res = ftruncate (fd, size);
   g_assert (res != -1);
 
+#ifdef HAVE_POSIX_FALLOCATE
   res = posix_fallocate (fd, 0, size);
   if (res != 0)
     {
       shm_unlink (name);
       g_error ("Not enough shared memory for window surface");
     }
+#endif
   
   ptr = mmap(0, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);