Applied 'epoch:' syntax to xrootd requirements in the slc6 part of cta.spec.in

cta.spec.in

@@ -33,9 +33,9 @@ BuildRequires: cmake >= 2.6 redhat-rpm-config
 # The CTA client is the only component of CTA that can be compiled on both SLC6
 # and C77, therefore only the packages it depends on are required for SLC6
 %if "%{?dist}" == ".slc6" || "%{?dist}" == ".el6"
-BuildRequires: xrootd-client-devel >= 4.2.3
-BuildRequires: xrootd-devel >= 4.2
-BuildRequires: cryptopp-devel >= 5.6.2
+BuildRequires: xrootd-client-devel = 1:4.2.3
+BuildRequires: xrootd-devel        = 1:4.2.3
+BuildRequires: cryptopp-devel = 1:5.6.2
 %else
 BuildRequires: xrootd-client-devel  = 1:4.4.1
 BuildRequires: xrootd-devel         = 1:4.4.1