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

Index: petsc/lib/petsc/conf/rules
===================================================================
--- petsc.orig/lib/petsc/conf/rules
+++ petsc/lib/petsc/conf/rules
@@ -170,7 +170,7 @@ clean-legacy:
 	       *.class *.ouit *.ad.* g_* silly.cmp *.tmp.* *.ilk *.pdb *.inst.c *.rej *.gcda *.gcno
 	@-${RM} -rf ${CLEANDIRS} *.dSYM AD_cache SunWS_cache
 
-clean:: clean-legacy
+clean: clean-legacy
 #
 #  Checks if directory requires particular package or language
 # The makefile may contain
Index: petsc/makefile
===================================================================
--- petsc.orig/makefile
+++ petsc/makefile
@@ -229,7 +229,7 @@ allclean:
 	   ${OMAKE} PETSC_ARCH=${PETSC_ARCH} PETSC_DIR=${PETSC_DIR} allclean-legacy; \
 	fi
 
-clean:: allclean
+clean: allclean
 
 distclean: chk_petscdir
 	@if [ -f ${PETSC_DIR}/${PETSC_ARCH}/lib/petsc/conf/reconfigure-${PETSC_ARCH}.py ]; then \