sb-grovel and the CC env var

  Hi,

when you try to load a system which uses sb-grovel, without a CC
environment variable set, SBCL breaks:

The CC environment variable has not been set in SB-GROVEL. Since this variable should always be set during
the SBCL build process, this might indicate an SBCL with a broken contrib installation.
   [Condition of type SIMPLE-ERROR]

Restarts:
 0: [RETRY] Retry compiling #<GROVEL-CONSTANTS-FILE "com.dvlsoft.clon" "termio" "sbcl" "constants">.
 1: [ACCEPT] Continue, treating compiling #<GROVEL-CONSTANTS-FILE "com.dvlsoft.clon" "termio"
"sbcl" "constants"> as having been successful.
 2: [RETRY] Retry SLIME REPL evaluation request.
 3: [*ABORT] Return to SLIME's top level.
 4: [REMOVE-FD-HANDLER] Remove #<SB-IMPL::HANDLER INPUT on descriptor 6: #<CLOSURE (LABELS
SWANK-BACKEND::RUN :IN SWANK-BACKEND:ADD-FD-HANDLER) {100403C5FB}>>
 5: [ABORT] Exit debugger, returning to top level.

Backtrace:
  0: ((SB-PCL::FAST-METHOD ASDF:PERFORM (ASDF:COMPILE-OP SB-GROVEL:GROVEL-CONSTANTS-FILE)) ..)
  1: ((SB-PCL::EMF ASDF:PERFORM) #<unavailable argument> #<unavailable argument> #<ASDF:COMPILE-OP
NIL {100497B403}> #<SB-GROVEL:GROVEL-CONSTANTS-FILE "com.dvlsoft.clon" "termio" "sbcl" "constants">)
  2: ((SB-PCL::FAST-METHOD ASDF::PERFORM-WITH-RESTARTS :AROUND (T T)) ..)
  3: ((LAMBDA () :IN ASDF::PERFORM-PLAN))
  4: ((FLET SB-THREAD::WITH-RECURSIVE-LOCK-THUNK :IN SB-C::%WITH-COMPILATION-UNIT))
  5: ((FLET SB-C::WITH-IT :IN SB-C::%WITH-COMPILATION-UNIT))
  6: ((SB-PCL::FAST-METHOD ASDF::PERFORM-PLAN (LIST)) ..)
  7: ((LAMBDA () :IN ASDF:OPERATE))