Allow gc_debug, redirect_malloc, large_config options in CMake script

Allow gc_debug, redirect_malloc, large_config options in CMake script

* CMakeLists.txt (enable_gc_debug, enable_redirect_malloc,
enable_large_config): New OPTION (off by default).
* CMakeLists.txt [enable_gc_debug] (DBG_HDRS_ALL, KEEP_BACK_PTRS):
Define macro.
* CMakeLists.txt [enable_redirect_malloc] (REDIRECT_MALLOC,
REDIRECT_REALLOC, REDIRECT_FREE, GC_USE_DLOPEN_WRAP): Likewise.
* CMakeLists.txt [enable_large_config] (LARGE_CONFIG): Likewise.
* CMakeLists.txt [enable_gc_debug] (MAKE_BACK_GRAPH): Define macro if
host is Linux or DG/UX.
* CMakeLists.txt [enable_gc_debug] (SAVE_CALL_COUNT): Define macro to 8
if host is Linux.
* CMakeLists.txt [enable_gc_debug] (SRC): Add backgraph.c if host is
Linux or DG/UX.