fix libdispatch.a

The static version (`libdispatch.a`) that can be compiled using
`./configure --enable static=yes` was missing all the Swift overlay symbols
(everything in `swift_overlay.o`). The reason for that is that the linker is
invoked through libtool which wants `.lo` files but the Swift overlay got passed
as a `.o` file. This first of all leads to this warning:

    *** Warning: Linking the shared library libdispatch.la against the non-libtool
    *** objects [...]/swift_overlay.o is not portable!

And the result is that libtool doesn't include `swift_overlay.o` when putting
together the static library `libdispatch.a`.

This patch fixes this problem is a pretty crude way. The real problem is that
libtool doesn't understand Swift. So it can't be used when compiling
`swift_overlay.o`. In order to still get a `.lo` file this patch tricks libtool
into generating one from the `swift_overlay.o` generated by `swiftc`. It's very
hacky but I'm not sure what else to do.

Signed-off-by: Daniel A. Steffen <dsteffen@apple.com>

Author: weissi

src/Makefile.am

@@ -157,12 +157,22 @@ SWIFT_SRC_FILES=\
 
 SWIFT_ABS_SRC_FILES = $(SWIFT_SRC_FILES:%=$(abs_srcdir)/%)
 SWIFT_OBJ_FILES = $(abs_builddir)/swift/swift_overlay.o
+SWIFT_LIBTOOL_OBJ_FILES = $(abs_builddir)/swift/swift_overlay.lo
 
 SWIFTC_FLAGS+= -Xcc -fmodule-map-file=$(abs_top_srcdir)/dispatch/module.modulemap -I$(abs_top_srcdir) -Xcc -fblocks
 if DISPATCH_ENABLE_OPTIMIZATION
 SWIFTC_FLAGS+=-O
 endif
 
+# this saves the object file, then tricks libtool into generating a .lo file and
+# then moves the object file back in the places libtool expects them to be for
+# the PIC and non-PIC case.
+$(abs_builddir)/swift/swift_overlay.lo:	$(abs_builddir)/swift/swift_overlay.o
+	mv $(abs_builddir)/swift/swift_overlay.o $(abs_builddir)/swift/.libs/swift_overlay.o.save
+	$(LIBTOOL) --mode=compile --tag=CC true -o $< -c /dev/null
+	cp $(abs_builddir)/swift/.libs/swift_overlay.o.save $(abs_builddir)/swift/.libs/swift_overlay.o
+	mv $(abs_builddir)/swift/.libs/swift_overlay.o.save $(abs_builddir)/swift/swift_overlay.o
+
 $(abs_builddir)/swift/swift_overlay.o:	$(SWIFT_ABS_SRC_FILES) $(SWIFTC)
 	@rm -f $@
 	$(SWIFTC) -whole-module-optimization -emit-library -c $(SWIFT_ABS_SRC_FILES) \
@@ -172,8 +182,8 @@ $(abs_builddir)/swift/swift_overlay.o:	$(SWIFT_ABS_SRC_FILES) $(SWIFTC)
 libdispatch_la_SOURCES+=swift/DispatchStubs.cc
 EXTRA_libdispatch_la_SOURCES+=$(SWIFT_SRC_FILES)
 
-EXTRA_libdispatch_la_DEPENDENCIES+=$(SWIFT_OBJ_FILES) $(abs_builddir)/swift/Dispatch.swiftmodule
-libdispatch_la_LIBADD+=$(SWIFT_OBJ_FILES)
+EXTRA_libdispatch_la_DEPENDENCIES+=$(SWIFT_OBJ_FILES) $(SWIFT_LIBTOOL_OBJ_FILES) $(abs_builddir)/swift/Dispatch.swiftmodule
+libdispatch_la_LIBADD+=$(SWIFT_LIBTOOL_OBJ_FILES)
 
 SWIFT_GEN_FILES=	\
 	$(abs_builddir)/swift/Dispatch.swiftmodule \