[7.6.8]

[7.6.8]

Bump gc version to 7.6.8

* ChangeLog (7.6.8): Set release date.
* Makefile.am (LIBGC_VER_INFO): Increment revision (change version info
of libgc.so to 4:4:3).
* Makefile.am (LIBGCCPP_VER_INFO): Increment revision (change version info
of libgccpp.so to 4:2:3).
* README.md: Bump micro version.
* configure.ac (AC_INIT): Likewise.
* include/gc_version.h (GC_TMP_VERSION_MICRO): Likewise.
* cord/cord.am (LIBCORD_VER_INFO): Increment revision (change version info
of libcord.so to 4:1:3).