root6: 10.8 build + portfile issues

[chrstphrchvz (Christopher Chavez)]

The 10.8 build has been failing. In some builds such as the ​most recent one (2020-09-18 11:01:24), C++14 code (usage of make_unique) is trying to be compiled while using -std=c++11:

In file included from /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_science_root6/root6/work/root-6-22-02/core/base/v7/src/RError.cxx:17:
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_science_root6/root6/work/root-6-22-02/core/base/v7/inc/ROOT/RError.hxx:142:55: error: no member named 'make_unique' in namespace 'std'
   explicit RResultBase(RError &&error) : fError(std::make_unique<RError>(std::move(error))) {}
                                                 ~~~~~^
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_science_root6/root6/work/root-6-22-02/core/base/v7/inc/ROOT/RError.hxx:142:67: error: 'RError' does not refer to a value
   explicit RResultBase(RError &&error) : fError(std::make_unique<RError>(std::move(error))) {}
                                                                  ^
/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_science_root6/root6/work/root-6-22-02/core/base/v7/inc/ROOT/RError.hxx:76:7: note: declared here
class RError {
      ^
2 errors generated.

There are possibly other minor issues in the portfile. Why not force using clang-mp-5.0 (which first gets added to compiler.blacklist) with configure.compiler rather than compiler.fallback? There is also how MacPorts' clang is normally be added as a runtime dependency for cling, however clang-mp-5.0 is not a version that will be used for this purpose. (It is potentially confusing to set the compiler after the compiler variables have already been read.)