> The output is stored in the build directory, not the source directory.

No, it's not.

    c-parse.o : $(srcdir)/c-parse.c $(CONFIG_H) $(TREE_H) c-lex.h $(GGC_H) intl.h \
        $(C_TREE_H) input.h flags.h system.h toplev.h output.h cpplib.h
        $(CC) $(ALL_CFLAGS) $(ALL_CPPFLAGS) $(INCLUDES) -c $(srcdir)/c-parse.c

    $(srcdir)/c-parse.c: $(srcdir)/c-parse.y
        (cd $(srcdir) && $(BISON) $(BISONFLAGS) -o c-p$$$$.c c-parse.y && \
        mv -f c-p$$$$.c c-parse.c)
    $(srcdir)/c-parse.y: c-parse.in
        echo '/*WARNING: This file is automatically generated!*/' >tmp-c-parse.y
        sed -e "/^ifobjc$$/,/^end ifobjc$$/d" \
            -e "/^ifc$$/d" -e "/^end ifc$$/d" \
            $(srcdir)/c-parse.in >>tmp-c-parse.y
        $(SHELL) $(srcdir)/move-if-change tmp-c-parse.y $(srcdir)/c-parse.y

    $(srcdir)/tradcif.c: $(srcdir)/tradcif.y
        (cd $(srcdir) && $(BISON) $(BISONFLAGS) -o tr$$$$.c tradcif.y && \
        mv -f tr$$$$.c tradcif.c)

It's all dumped into the same srcdir where the rest of the .c files live. That's how it's always been. Also, PCC does it about the same way:

    srcdir=.
    top_srcdir=../..

    cgram.c: $(srcdir)/cgram.y
        $(YACC) $(YFLAGS) -d $(srcdir)/cgram.y
        mv -f y.tab.c cgram.c
        mv -f y.tab.h cgram.h

    scan.c: $(srcdir)/scan.l
        $(LEX) $(LFLAGS) $(srcdir)/scan.l
        mv -f $(LEX_OUTPUT_ROOT).c scan.c

    $(OBJS): $(HDRS) external.c cgram.c

    cgram.o: cgram.c
        $(CC) $(CF0) $(CFLAGS) $(CPPFLAGS) -c -o $@ cgram.c

    scan.o: scan.c
        $(CC) $(CF0) $(CFLAGS) $(CPPFLAGS) -c -o $@ scan.c