*-wrapper; Switch from infixSalt to suffixSalt

I hate the thing too even though I made it, and rather just get rid of
it. But we can't do that yet. In the meantime, this brings us more
inline with autoconf and will make it slightly easier for me to write a
pkg-config wrapper, which we need.

pkgs/development/compilers/llvm/9/libc++/setup-hook.sh

@@ -2,5 +2,5 @@
 getHostRole
 
 linkCxxAbi="@linkCxxAbi@"
-export NIX_${role_pre}CXXSTDLIB_COMPILE+=" -isystem @out@/include/c++/v1"
-export NIX_${role_pre}CXXSTDLIB_LINK=" -stdlib=libc++${linkCxxAbi:+" -lc++abi"}"
+export NIX_CXXSTDLIB_COMPILE${role_post}+=" -isystem @out@/include/c++/v1"
+export NIX_CXXSTDLIB_LINK${role_post}=" -stdlib=libc++${linkCxxAbi:+" -lc++abi"}"