Support header files installation in CMake script

Support header files installation in CMake script

Issue #281 (bdwgc).

Now, "cmake --target install" copies the public header files (to the
installation destination) unless -Dinstall_headers=OFF is passed to
cmake.

* CMakeLists.txt (install_headers): New option (on by default).
* CMakeLists.txt [install_headers] (include/gc.h, include/gc_backptr.h,
include/gc_config_macros.h, include/gc_gcj.h, include/gc_inline.h,
include/gc_mark.h, include/gc_pthread_redirects.h,
include/gc_tiny_fl.h, include/gc_typed.h, include/gc_version.h,
include/javaxfc.h, include/leak_detector.h): Specify install to
"include/gc" folder.
* CMakeLists.txt [install_headers && enable_cplusplus]
(include/gc_allocator.h, include/gc_cpp.h): Likewise.
* CMakeLists.txt [install_headers && enable_disclaim]
(include/gc_disclaim.h): Likewise.
* CMakeLists.txt [install_headers] (include/extra/gc.h): Specify
install to "include" folder.
* CMakeLists.txt [install_headers && enable_cplusplus]
(include/extra/gc_cpp.h): Likewise.