Fix build errors with CHPL_DEVELOPER set.

https://github.com/chapel-lang/chapel/pull/194 causes build errors when
CHPL_DEVELOPER is set.  Fix those:
- Move the extern declaration for environ to the file scope.  It's still
  duplicates the one in <unistd.h> in certain environments, but gcc
  doesn't gripe if the duplicate isn't in a nested scope.
- Cast the assignment of "-E" to (char*) at line 62; by default a string
  constant has type (const char*), but the left side in this case isn't
  const.