Dynamic only GHC

[Jeremy]

I'm trying to build a minimal GHC 7.8.3 on Debian Wheezy with only dynamic
libraries (this is for a system where disc space is scarce). I'm using this
build.mk:

GhcRTSWays = thr
GhcLibWays = dyn
HADDOCK_DOCS = NO
DYNAMIC_BY_DEFAULT = YES
GhcDynamic = YES

Tried with and without GhcDynamic (asked on beginners how it's different to
DYNAMIC_GHC_PROGRAMS but still waiting for an answer). It gets near the end
of the build, then fails with:

  HC [stage 1] ghc/stage2/build/Main.dyn_o
  HC [stage 1] ghc/stage2/build/tmp/ghc-stage2
/usr/bin/ld: cannot find -lHSrts_thr-ghc7.8.3
collect2: error: ld returned 1 exit status
make[1]: *** [ghc/stage2/build/tmp/ghc-stage2] Error 1
make: *** [all] Error 2

If I build with only static libraries, everything seems to work OK.