Clean the build root for older systems only

As per:
* http://fedoraproject.org/wiki/Packaging:Guidelines#.25clean
* http://fedoraproject.org/wiki/EPEL:Packaging#Prepping_BuildRoot_For_.25install
* http://fedoraproject.org/wiki/EPEL:Packaging#Cleaning_BuildRoot_in_.25clean

The removal of the `%{buildroot}` directory should be done in the
`%clean` and `%install` sections, but only for RHEL5 and earlier.

package/rpm/pcp-pmda-cpp.spec

@@ -25,7 +25,6 @@ Metrics Domain Agents (PMDAs) for Performance Co-Pilot (PCP) in C++.
 %setup -c -q
 
 %build
-%{__rm} -rf %{buildroot}
 %{__mkdir_p} build/devel
 pushd build/devel
 %cmake ../../pcp-pmda-cpp-%{version}/include
@@ -38,11 +37,16 @@ pushd build/examples
 popd
 
 %install
+%if 0%{?rhel} < 6
+%{__rm} -rf %{buildroot}
+%endif
 %{__make} install DESTDIR=%{buildroot} -C build/devel
 %{__make} install DESTDIR=%{buildroot} -C build/examples
 
 %clean
+%if 0%{?rhel} < 6
 %{__rm} -rf %{buildroot}
+%endif
 
 %package devel
 Summary: Development headers for the PMDA++ library