libpng12: resolve conflicts with libpng{,-devel}

fixes #4959.

srcpkgs/libpng12/template

@@ -1,7 +1,7 @@
 # Template build file for 'libpng12'.
 pkgname=libpng12
 version=1.2.56
-revision=1
+revision=2
 wrksrc=libpng-${version}
 build_style=gnu-configure
 makedepends="zlib-devel"
@@ -15,11 +15,13 @@ checksum=24ce54581468b937734a6ecc86f7e121bc46a90d76a0d948dca08f32ee000dbe
 post_install() {
 	rm -f ${DESTDIR}/usr/lib/*.la
 	vlicense LICENSE
+	mv $DESTDIR/usr/share/man/man5/png.5 $DESTDIR/usr/share/man/man5/png12.5
 }
 
 libpng12-devel_package() {
 	depends="${makedepends} libpng12>=${version}_${revision}"
 	short_desc+=" - development files"
+	conflicts="libpng-devel"
 	pkg_install() {
 		vmove "usr/bin/libpng*-config"
 		vmove usr/include