Support configure --disable-thread-local-alloc option (similar for CMake)

Support configure --disable-thread-local-alloc option (similar for CMake)

* CMakeLists.txt (enable_thread_local_alloc): New option (on by
default).
* CMakeLists.txt [CMAKE_USE_PTHREADS_INIT
|| CMAKE_USE_WIN32_THREADS_INIT] (SRC): Add thread_local_alloc.c only
if enable_thread_local_alloc.
* CMakeLists.txt [CMAKE_USE_PTHREADS_INIT
|| CMAKE_USE_WIN32_THREADS_INIT]: Define THREAD_LOCAL_ALLOC macro only
if enable_thread_local_alloc.

* Makefile.am (libgc_la_SOURCES): Add thread_local_alloc.c only if
THREAD_LOCAL_ALLOC.
* configure.ac (thread-local-alloc): New option.
* configure.ac: AC_DEFINE(THREAD_LOCAL_ALLOC) only if
enable_thread_local_alloc is yes or unset.
* configure.ac (THREAD_LOCAL_ALLOC): New AM_CONDITIONAL.