Plumb --with-gcc= value through to GHC.

Previously, the c compiler selected by the user was only being used in
the early stages (preprocessing, perhaps another).  It seems obvous that
a user specifying --with-gcc=clang would want C code, such as any values
from cbits/*, to be compiled using clang.  We shoud likely plumb other
compiler options through as well, but doing so in a clean manner is
another issue.

Also, notice the option should be '--with-cc' someday.