Disable vala, as it was disabled upstream:

http://git.gnome.org/browse/libpeas/commit/?id=1031aaeeef282ab2bb65cb6ae48fa4abff453c4d

libpeas.spec

@@ -26,7 +26,6 @@ BuildRequires:	libtool
 BuildRequires:	seed-devel
 %endif
 BuildRequires:	gjs-devel
-BuildRequires:	vala-devel
 BuildRequires:	gtk-doc
 BuildRequires:	glade-devel
 
@@ -54,7 +53,7 @@ that are needed to write applications that use libpeas.
 %setup -q
 
 %build
-%configure --enable-vala %{seed_option}
+%configure --disable-vala %{seed_option}
 
 make %{?_smp_mflags}
 
@@ -108,6 +107,10 @@ gtk-update-icon-cache %{_datadir}/icons/hicolor >&/dev/null || :
 %{_datadir}/glade/catalogs/libpeas-gtk.xml
 
 %changelog
+* Wed Sep 19 2012 Bastien Nocera <bnocera@redhat.com> 1.5.0-1
+- Disable vala, as it was disabled upstream:
+http://git.gnome.org/browse/libpeas/commit/?id=1031aaeeef282ab2bb65cb6ae48fa4abff453c4d
+
 * Wed Jul 18 2012 Ignacio Casal Quinteiro <icq@gnome.org> - 1.5.0-1
 - Update to 1.5.0