skype: build qt4 with the 32bits clangStdenv

This allows to build skype without errors like this:
clang++ -Wl,-O1 -o libmng libmng.o     -lmng
/nix/store/1d2abmmqvindckyq51nq9kd1yisiv54j-binutils-2.26/bin/ld: skipping incompatible /nix/store/72nklasrjg774iwxxnpyxwkzxz2j37v5-libmng-2.0.2/lib/libmng.so when searching for -lmng
/nix/store/1d2abmmqvindckyq51nq9kd1yisiv54j-binutils-2.26/bin/ld: cannot find -lmng
/nix/store/1d2abmmqvindckyq51nq9kd1yisiv54j-binutils-2.26/bin/ld: skipping incompatible /nix/store/n0y8pv4kaff7vnq7rmzwrd654gqaj0ki-glibc-2.23/lib/libm.so when searching for -lm
/nix/store/1d2abmmqvindckyq51nq9kd1yisiv54j-binutils-2.26/bin/ld: skipping incompatible /nix/store/n0y8pv4kaff7vnq7rmzwrd654gqaj0ki-glibc-2.23/lib/libm.a when searching for -lm
clang-3.7: error: linker command failed with exit code 1 (use -v to see invocation)