eliminate a bit of duplication

libraries/Makefile

@@ -353,7 +353,7 @@ $(foreach SUBDIR,$(SUBDIRS),install.library.$(SUBDIR)): \
 install.library.%: installPackage/installPackage ifBuildable/ifBuildable
 	if ifBuildable/ifBuildable $*; then \
 	  cd $* && \
-	  ../installPackage/installPackage '$(GHC_PKG_INPLACE)' '$(DESTDIR)$(libdir)/package.conf' '$(DESTDIR)' '$(prefix)' '$(iprefix)' '$(ibindir)' '$(ilibdir)' '$(ilibexecdir)' '$(idatadir)' '$(idocdir)' '$(ihtmldir)' '$(iinterfacedir)' ; \
+	  ../installPackage/installPackage '$(GHC_PKG_PROG)' '$(DESTDIR)$(libdir)/package.conf' '$(DESTDIR)' '$(prefix)' '$(iprefix)' '$(ibindir)' '$(ilibdir)' '$(ilibexecdir)' '$(idatadir)' '$(idocdir)' '$(ihtmldir)' '$(iinterfacedir)' ; \
 	fi
 
 .PHONY: binary-dist binary-dist.library.%

mk/config.mk.in

@@ -611,6 +611,11 @@ ifacedir		= $(libdir)
 #     (NOTE: configure script setting is ignored).
 libexecdir		= $(libdir)
 
+# This is a bit of a lie, as this is a wrapper rather than the program
+# itself. However, it means that we don't have to worry about Windows
+# and non-Windows having different extensions.
+GHC_PKG_PROG = $(FPTOOLS_TOP_ABS)/$(GHC_PKG_DIR_REL)/ghc-pkg-inplace
+
 #-----------------------------------------------------------------------------
 # install configuration