Add --with-scheme48 option to ./configure, to indicate scheme48 location

This also checks that we can find scheme48.h, and bombs out if not.
Merge pull request #3 from nxg/add-config-option

Add --with-scheme48 option to ./configure, to indicate scheme48 location
Initial port of scsh-time package. Few things more to do before this works.

Further work needed:
- change time.c and time.h to use the new ffi
- define ticks/sec in a operating system independent way.
Whitespace fixes in time.scm.
Use the new ffi in time.c and time.h. Fix ticks/sec.
Remove unused code in time.scm.
This fixes here strings, but is obviously not ideal.

Trying to back out into more sane changes from this point.
Have delimited-readers not depend on rx.
More sane fix for here strings.

I don't understand.. this didn't work earlier.
Remove mode line in scsh-read.scm.
Remove duplication in let-opt.scm
Update to latest rx revision. Fixes #5.
Make the definition of ttyout/all-delay tolerant to missing flags.
Fix calls related to opening files.

They failed to align certain process state before being called,
including the the cwd.
Formatting and addressing clang warnings in scsh-tramp.c.
Addressing clang warnings in syscalls.c.
Placation of clang warnings in time.c.
Fix mangling of environment variables for child processes.

Can't keep references to the result of s48_extract_byte_vector_2. Will likely
be invalid after function returns. malloc and copy instead in create_env().

Use s48_enter_byte_vector_2 instead of s48_enter_string_latin_1_2 when
copying original value of environ.
Fix redirects.

Formatting changes to make TRANSCRIBE-REDIRECTION and related more readable.
Add ERROR and WARN to the scsh interface.

This adds them to scsh-with-scheme and scsh-user as well, so they'll be
available at the default repl. Fixes #4.