SYScall on a pipe a | b only fails if b fails. (shell semantics) Since
the preprocessor call also invokes sed, the entire command does appear
to succeed even if preprocessing fails.

Overview:

- potentially separate output directories for Tree and Mod files.
  Option -o sets output dir for Tree files, -olib for Mod files, but
  flag -olib defaults to be the same as -o if not specified.

- flag -install for modules defaults the output directories set
  by new resources TREE_OUTPUTDIR and LIB_OUTPUTDIR.

This patch introduces the notion of "SaC Binary Interface" as discussed
during the 2013 DevCamp. An overview of the approach is given in the
"Cross-compilation" document written during the DevCamp, available
separately.

Outline of user-facing changes:

- updated explanatory comments at the start of sac2crc.

- new resources in sac2crc.

- the command-line options -mt, -mtmode disappear in favor of
  sac2crc resources.

- '-numthread 1' does not disable -mt any more

- new command-line options -Xc, -Xtc, -Xl, -Xtl, -T, -cc

- tree files are searched in TREEPATH, module code is searched
  in LIBPATH (previously LIBPATH was searched for both)

- "make install" / "make uninstall" works. See the hidden file
  ".uninstall" at the root of the source tree after "make install"
  for an overview of installed files.

Outine of developer-facing changes:

- the sac2c modules CCM and LIBB are merged into one module CCT.

- sac2c is now used to compile libsac, libsacphm and libsacphmc.  The
  SBI settings defined by sac2crc are propagated to the
  libsac/libsacphm/libsacphmc code by means of preprocessor defines:
  -DSAC_BACKEND_xxx, -DSAC_MT_MODE, -DSAC_MT_LIB_xxxx, -DSAC_DO_RTSPEC.

- libsac2c does not have features conditional on the build environment
  any more. Only features of libsac/libsacphm/libsacphmc (which are
  running on the target environment) can be conditionally included.