It looks like changing the order does not help.  When building `OpenMPI-1.6.4-ClangGCC-1.1.2.eb`:

    == 2013-04-10 02:43:11,987 main.ClangGcc DEBUG _setenv_variables: setting environment variable CC to gcc
    == 2013-04-10 02:43:11,988 main.environment INFO Environment variable CC set to gcc
    == 2013-04-10 02:43:11,988 main.environment INFO Environment variable EBVARCC set to gcc