Patching works better if you use patch.

PKGBUILD

@@ -4,7 +4,7 @@
 
 pkgname=lxsession
 pkgver=0.4.6.1
-pkgrel=2
+pkgrel=3
 pkgdesc='Lightweight X11 session manager'
 arch=('i686' 'x86_64')
 url="http://lxde.org/"
@@ -21,6 +21,7 @@ md5sums=('e456b64c5ab3652a4dec661ec706dc6e'
 
 build() {
   cd $srcdir/$pkgname-$pkgver
+  patch -Np2 -i $srcdir/lxsession-0.4.6.1-logind-support.patch
   ./configure --sysconfdir=/etc --prefix=/usr
   make
 }