Add configure option to compile all library .c files into single gc.o

Add configure option to compile all library .c files into single gc.o

* Makefile.am (SINGLE_GC_OBJ): Test new AM conditional.
* Makefile.am (libgc_la_SOURCES): Put only extra/gc.c entry here if
SINGLE_GC_OBJ.
* Makefile.am (EXTRA_DIST): Add extra/gc.c only if not SINGLE_GC_OBJ.
* configure.ac (single-obj-compilation): New AC "enable" argument
(turning on compilation of single extra/gc.c file instead of all .c
files from GC base folder).
* configure.ac (SINGLE_GC_OBJ): New AM conditional.