Enable memory unmapping by default

Enable memory unmapping by default

Issue #152 (bdwgc).

* CMakeLists.txt (enable_munmap): New OPTION (on by default).
* CMakeLists.txt [enable_munmap] (USE_MMAP, USE_MUNMAP): Define macro.
* CMakeLists.txt [enable_checksums && enable_munmap): Issue MESSAGE
that CHECKSUMS is not compatible with USE_MUNMAP.
* configure.ac (AC_ARG_ENABLE(munmap)): Update the help message.
* configure.ac [$enable_munmap=""]: Treat as enable_munmap="yes".
* configure.ac [$MUNMAP_THRESHOLD=""]: Treat as MUNMAP_THRESHOLD="yes".
* doc/README.win32 (GNU Tools): Note about "--disable-munmap" option.
* doc/debugging.md (Unexpectedly Large Heap): Remove advice about
"--enable-munmap" option.