Remove double-colon from clean target which causes breakage in reverse-depends.

Index: petsc-3.4.2/conf/rules
===================================================================
--- petsc-3.4.2.orig/conf/rules	2013-08-20 15:33:06.000000000 +0200
+++ petsc-3.4.2/conf/rules	2013-08-20 15:33:06.000000000 +0200
@@ -168,7 +168,7 @@
 	@${CP} -f *.mod ${PETSC_DIR}/${PETSC_ARCH}/include
 
 # Removes garbage files
-clean::
+clean:
 	@-${RM} ${CLEANFILES} ${TESTS} *.o *.lo *~ \
                ex[0-9] ex[0-9][0-9] ex[0-9][0-9][0-9] \
                ex[0-9]f ex[0-9][0-9]f ex[0-9][0-9][0-9]f \