Remove references to imake.

The original imake file was not included in the source tarball from the
author's site, only the generated Makefile. Replace all references to
"imake" with "make", and remove the "do not edit" warning at the top of
the file.
Reword comment explaining the $GSC var.
Add comment about YOU_HAVE_TIME and "long" tests.
Remove duplicate comments from */*-book.scm files.

Only provide full comments in mitscheme/mit-book.scm, and mention that
file in all other *-book.scm files.
Export and use $CC for all compilations.

Use $CC to compile generated C code in the chap6f and chap10f compilers,
rather than hard coding "gcc".  This allows overriding CC, which is
useful when compiling on FreeBSD, where "gcc" is named gccXY (where XY
is a version number).