The bkpcabal01 testcase introduced the by Backpack merge fails on Mac OS X with,

=====> bkpcabal01(normal) 1 of 1 [0, 0, 0]
cd "./backpack/cabal/bkpcabal01/bkpcabal01.run" && $MAKE -s --no-print-directory bkpcabal01 CLEANUP=1  
Actual stderr output differs from expected:
--- /dev/null	2016-10-08 22:51:23.000000000 +0300
+++ ./backpack/cabal/bkpcabal01/bkpcabal01.run/bkpcabal01.run.stderr.normalised	2016-10-08 22:51:23.000000000 +0300
@@ -0,0 +1,4 @@
+/Applications/Xcode-7.2/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: dist/build/p-0.1+FBOSaiWyMx9DR2UZVI6wQJ/objs-36887/libHSp-0.1+FBOSaiWyMx9DR2UZVI6wQJ.a(H.o) has no symbols
+/Applications/Xcode-7.2/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: dist/build/q-0.1+70e5o6lPGfgGiochTG2tqQ/objs-36936/libHSq-0.1+70e5o6lPGfgGiochTG2tqQ.a(I.o) has no symbols
+/Applications/Xcode-7.2/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: dist/build/p-0.1+FBOSaiWyMx9DR2UZVI6wQJ/objs-37078/libHSp-0.1+FBOSaiWyMx9DR2UZVI6wQJ.a(H.o) has no symbols
+/Applications/Xcode-7.2/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: dist/build/q-0.1+70e5o6lPGfgGiochTG2tqQ/objs-37123/libHSq-0.1+70e5o6lPGfgGiochTG2tqQ.a(I.o) has no symbols
*** unexpected failure for bkpcabal01(normal)

It seems that the problem is that ar produces a warning when producing an archive file containing an object file with no symbols,

bkpcabal01 bgamari$ make SETUP="./Setup -v3" bkpcabal01
...
("/usr/bin/ar",["-r","-s","-v","dist/build/q-0.1+70e5o6lPGfgGiochTG2tqQ/objs-39951/libHSq-0.1+70e5o6lPGfgGiochTG2tqQ.a","dist/build/q-0.1+70e5o6lPGfgGiochTG2tqQ/Q.o","dist/build/q-0.1+70e5o6lPGfgGiochTG2tqQ/I.o"])
ar: creating archive dist/build/q-0.1+70e5o6lPGfgGiochTG2tqQ/objs-39951/libHSq-0.1+70e5o6lPGfgGiochTG2tqQ.a
a - dist/build/q-0.1+70e5o6lPGfgGiochTG2tqQ/Q.o
a - dist/build/q-0.1+70e5o6lPGfgGiochTG2tqQ/I.o
/Applications/Xcode-7.2/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: dist/build/q-0.1+70e5o6lPGfgGiochTG2tqQ/objs-39951/libHSq-0.1+70e5o6lPGfgGiochTG2tqQ.a(I.o) has no symbols

In general it would be best if we could avoid linking these empty object files to avoid these spurious warnings.