This patch could be replaced by a configure call if
http://hackage.haskell.org/trac/ghc/ticket/5743 were fixed.

Index: ghc-7.6.1/rts/package.conf.in
===================================================================
--- ghc-7.6.1.orig/rts/package.conf.in	2012-09-04 19:10:15.000000000 +0200
+++ ghc-7.6.1/rts/package.conf.in	2012-10-08 13:06:55.167887121 +0200
@@ -24,8 +24,9 @@
 hs-libraries:   "HSrts"
 
 extra-libraries:
+				"ffi"
 #ifdef HAVE_LIBM
-                		"m"		/* for ldexp() */
+			      , "m"		/* for ldexp() */
 #endif
 #ifdef HAVE_LIBRT
 			      , "rt"
Index: ghc-7.6.1/ghc.mk
===================================================================
--- ghc-7.6.1.orig/ghc.mk	2012-09-04 19:10:15.000000000 +0200
+++ ghc-7.6.1/ghc.mk	2012-10-08 13:06:55.171887120 +0200
@@ -600,7 +600,6 @@
    $(MAYBE_GHCI) \
    driver/ghc \
    driver/haddock \
-   libffi \
    includes \
    rts
 
Index: ghc-7.6.1/rts/ghc.mk
===================================================================
--- ghc-7.6.1.orig/rts/ghc.mk	2012-09-04 19:10:15.000000000 +0200
+++ ghc-7.6.1/rts/ghc.mk	2012-10-08 13:08:52.019882891 +0200
@@ -177,12 +177,12 @@
 # Making a shared library for the RTS.
 ifneq "$$(findstring dyn, $1)" ""
 ifeq "$$(HostOS_CPP)" "mingw32" 
-$$(rts_$1_LIB) : $$(rts_$1_OBJS) $$(ALL_RTS_DEF_LIBS) rts/libs.depend rts/dist/build/$$(LIBFFI_DLL)
+$$(rts_$1_LIB) : $$(rts_$1_OBJS) $$(ALL_RTS_DEF_LIBS) rts/libs.depend
 	"$$(RM)" $$(RM_OPTS) $$@
 	"$$(rts_dist_HC)" -package-name rts -shared -dynamic -dynload deploy \
 	  -no-auto-link-packages -Lrts/dist/build -l$(LIBFFI_WINDOWS_LIB) `cat rts/libs.depend` $$(rts_$1_OBJS) $$(ALL_RTS_DEF_LIBS) -o $$@
 else
-$$(rts_$1_LIB) : $$(rts_$1_OBJS) $$(rts_$1_DTRACE_OBJS) rts/libs.depend rts/dist/build/libffi$$(soext)
+$$(rts_$1_LIB) : $$(rts_$1_OBJS) $$(rts_$1_DTRACE_OBJS) rts/libs.depend
 	"$$(RM)" $$(RM_OPTS) $$@
 	"$$(rts_dist_HC)" -package-name rts -shared -dynamic -dynload deploy \
 	  -no-auto-link-packages -Lrts/dist/build -lffi `cat rts/libs.depend` $$(rts_$1_OBJS) \
@@ -193,9 +193,9 @@
 endif
 endif
 else
-$$(rts_$1_LIB) : $$(rts_$1_OBJS) $$(rts_$1_DTRACE_OBJS) $$(rts_ffi_objs_stamp)
+$$(rts_$1_LIB) : $$(rts_$1_OBJS) $$(rts_$1_DTRACE_OBJS)
 	"$$(RM)" $$(RM_OPTS) $$@
-	echo $$(rts_ffi_objs) $$(rts_$1_OBJS) $$(rts_$1_DTRACE_OBJS) | "$$(XARGS)" $$(XARGS_OPTS) "$$(AR_STAGE1)" \
+	echo $$(rts_$1_OBJS) $$(rts_$1_DTRACE_OBJS) | "$$(XARGS)" $$(XARGS_OPTS) "$$(AR_STAGE1)" \
 		$$(AR_OPTS_STAGE1) $$(EXTRA_AR_ARGS_STAGE1) $$@
 endif
 
@@ -509,10 +509,8 @@
 # installing
 
 INSTALL_LIBS += $(ALL_RTS_LIBS)
-INSTALL_LIBS += $(wildcard rts/dist/build/libffi$(soext)*)
-INSTALL_LIBS += $(wildcard rts/dist/build/$(LIBFFI_DLL))
 
-install: install_libffi_headers
+install:
 
 .PHONY: install_libffi_headers
 install_libffi_headers :