Disable static linking to libstdc++ on RHEL6

MantidPlot no longer starts when this is combined with more recent changes to the code. It also turns out that the gcc provided with devtoolset includes modifications to statically link newer libstdc++ symbols that are not present in the  system version so that the binaries will run on legacy systems without the devtoolset.

buildconfig/Jenkins/buildscript

@@ -123,9 +123,9 @@ fi
 ###############################################################################
 # RHEL6 setup steps - nodes must have a "rhel6" label set (in lowercase)
 ###############################################################################
+DIST_FLAGS="-DSTATIC_LIBSTDCXX=OFF"
 if [[ ${NODE_LABELS} == *rhel6* ]]; then
   SCL_ON_RHEL6="scl enable mantidlibs34 devtoolset-2"
-  DIST_FLAGS="-DSTATIC_LIBSTDCXX=ON"
   ON_RHEL6=true
 else
   SCL_ON_RHEL6="eval"