sundials @5.8.0 won't install on < 10.8 because mpich-clang12 doesn't exist

[kencu (Ken)]

the default compiler moved up on older systems, but the mpich ports and the mpi Portgroup haven't included clang-12 yet.

That leads to failures such as:

​https://build.macports.org/builders/ports-10.7_x86_64-builder/builds/83561/steps/install-port/logs/stdio

--->  Configuring sundials
DEBUG: Preferred compilers: macports-clang-12 macports-clang-11 macports-clang-10 macports-clang-9.0 macports-clang-8.0 macports-clang-7.0 macports-clang-6.0 macports-clang-5.0 macports-clang-13 macports-clang-3.7 macports-clang-3.4
DEBUG: Using compiler 'MacPorts Clang 12'
DEBUG: Executing proc-pre-org.macports.configure-configure-0
DEBUG: CPPFLAGS="-I/opt/local/include" inserted into CFLAGS="-Os -DNDEBUG -I/opt/local/include" CXXFLAGS="-Os -DNDEBUG -I/opt/local/include -stdlib=libc++"
DEBUG: Executing proc-pre-org.macports.configure-configure-1
DEBUG: Executing proc-pre-org.macports.configure-configure-2
DEBUG: Active variants check for source-type install considers depends_fetch depends_extract depends_lib depends_build depends_run: SuiteSparse_KLU libgcc cmake gcc11 clang-12
DEBUG: Executing proc-pre-org.macports.configure-configure-3
DEBUG: compilers.enforce_c list: 
DEBUG: compilers.enforce_fortran list: 
DEBUG: compilers.enforce_some_fortran list: 
DEBUG: Executing proc-pre-org.macports.configure-configure-4
Error: active_variants: Error: invalid port depspec ''
Error:   expecting either: port or (bin|lib|path):foo:port
Error: active_variants: Error: invalid port depspec ''
Error:   expecting either: port or (bin|lib|path):foo:port
Error: active_variants: Error: invalid port depspec ''
Error:   expecting either: port or (bin|lib|path):foo:port
Error: active_variants: Error: invalid port depspec ''
Error:   expecting either: port or (bin|lib|path):foo:port
Error: active_variants: Error: invalid port depspec ''
Error:   expecting either: port or (bin|lib|path):foo:port
Error: active_variants: Error: invalid port depspec ''
Error:   expecting either: port or (bin|lib|path):foo:port
Error: active_variants: Error: invalid port depspec ''
Error:   expecting either: port or (bin|lib|path):foo:port
Error: active_variants: Error: invalid port depspec ''
Error:   expecting either: port or (bin|lib|path):foo:port
Error: active_variants: Error: invalid port depspec ''
Error:   expecting either: port or (bin|lib|path):foo:port
Error: active_variants: Error: invalid port depspec ''
Error:   expecting either: port or (bin|lib|path):foo:port
Error:  has a different Fortran variant () than the one selected, gcc11 (via +gfortran).
Error: Failed to configure sundials: Install  +gcc11
DEBUG: Error code: NONE
DEBUG: Backtrace: Install  +gcc11
    while executing
"$pre $targetname"
Error: See /opt/local/var/macports/logs/_opt_bblocal_var_buildworker_ports_build_ports_math_sundials/sundials/main.log for details.
Error: Follow https://guide.macports.org/#project.tickets if you believe there is a bug.
Error: Processing of port sundials failed
Build of 'sundials' failed.