gnu: gcc: Add a "lib" output.

* gnu/packages/gcc.scm (gcc-4.7)[configure-flags]: Add
  --with-gxx-include-dir.
  [outputs]: New field.
  [arguments] <phases>: Adjust to use the "lib" output.  Add
  'substitute*' calls for "libstdc++-v3/python/Makefile.in" and
  "gcc/config.in".
* gnu/packages/base.scm (gcc-boot0) <symlink-libgcc_eh>: Use the "lib"
  output.
  (cross-gcc-wrapper): Add 'outputs' field.
  (glibc-final)[arguments]: Adjust #:allowed-references to list
  '(GCC-BOOT0 "lib").
  (libstdc++): Add 'outputs' field.
  (gcc-final)[arguments]: Add "lib" to #:allowed-references.
* gnu/packages/make-bootstrap.scm (%gcc-static): Add 'outputs' field.