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

Index: ghc-7.4.0.20111219/ghc.mk
===================================================================
--- ghc-7.4.0.20111219.orig/ghc.mk	2011-12-21 23:21:03.000000000 +0100
+++ ghc-7.4.0.20111219/ghc.mk	2011-12-21 23:21:04.000000000 +0100
@@ -579,7 +579,6 @@
    driver/ghci \
    driver/ghc \
    driver/haddock \
-   libffi \
    includes \
    rts
 
Index: ghc-7.4.0.20111219/rts/ghc.mk
===================================================================
--- ghc-7.4.0.20111219.orig/rts/ghc.mk	2011-12-21 19:56:29.000000000 +0100
+++ ghc-7.4.0.20111219/rts/ghc.mk	2011-12-21 23:23:52.000000000 +0100
@@ -177,7 +176,7 @@
 	"$$(rts_dist_HC)" -package-name rts -shared -dynamic -dynload deploy \
 	  -no-auto-link-packages -Lrts/dist/build -lffi-5 `cat rts/libs.depend` $$(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) \
@@ -188,9 +187,9 @@
 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
 
@@ -504,10 +503,8 @@
 # installing
 
 INSTALL_LIBS += $(ALL_RTS_LIBS)
-INSTALL_LIBS += $(wildcard rts/dist/build/libffi$(soext)*)
-INSTALL_LIBS += $(wildcard rts/dist/build/libffi-5.dll)
 
-install: install_libffi_headers
+install:
 
 .PHONY: install_libffi_headers
 install_libffi_headers :