bkpcabal01 testcase fails with spurious output on stderr on Darwin

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.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Package system
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	ezyang
Operating system
Architecture