Set LIB_INSTALL_DIR for cmake-based projects

[staticfloat]

This fixes MULTIARCH_INSTALL=1 installations that place libraries not
into <prefix>/lib but into <prefix>/lib/<host-triplet>, like Debian
and Ubuntu. Without this change, libgit2, libssh2 and mbedtls
would place libraries into <prefix>/lib, silently failing until the
Julia bootstrap process attempted to dlopen one of them.

This is at least one problem that has held up the Ubuntu nightlies.

[staticfloat]

Confirmed that this has allowed a debuild on a local Ubuntu machine to go through all the relevant tests (it still failed on a numbers test), so that's a good sign.

For reference, the numbers failure was the following:

From worker 9:       * numbers              Error During Test
        From worker 9:    Test threw an exception of type DivideError
        From worker 9:    Expression: powermod(1,-1,big(1)) == 0
        From worker 9:    DivideError: integer division error
        From worker 9:     in powermod(::BigInt, ::BigInt, ::BigInt) at ./gmp.jl:442
        From worker 9:     in powermod(::Int64, ::Int64, ::BigInt) at ./gmp.jl:448
        From worker 9:     in include_string(::String, ::String) at ./loading.jl:380
        From worker 9:     in include_from_node1(::String) at ./loading.jl:429
        From worker 9:     in macro expansion at ./util.jl:226 [inlined]
        From worker 9:     in runtests(::String) at /tmp/julianightlies/test/testdefs.jl:7
        From worker 9:     in (::Base.Serializer.__deserialized_types__.##16#24)(::String)
 at /tmp/julianightlies/test/runtests.jl:44
        From worker 9:     in (::Base.##585#587{Base.CallMsg{:call_fetch}})() at ./multi.j
l:1410
        From worker 9:     in run_work_thunk(::Base.##585#587{Base.CallMsg{:call_fetch}},
::Bool) at ./multi.jl:996
        From worker 9:     in macro expansion at ./multi.jl:1410 [inlined]
        From worker 9:     in (::Base.##584#586{Base.CallMsg{:call_fetch},Base.MsgHeader,T
CPSocket})() at ./event.jl:68

[tkelman]

We aren't building LLVM with cmake by default yet, but would this be needed there if we did?

[staticfloat]

Almost certainly.

[tkelman]

Why not put this in CMAKE_COMMON then?

[staticfloat]

One thing to think about before I do that; on Windows we set $(build_shlibdir) to $(build_bindir) because .dll's should live with the .exe's, but we still want the static libraries (e.g. $(build_libdir)) to be separate, right? Should I then be setting -DLIB_INSTALL_DIR=$(build_libdir) and then we hope that our installation rules are smart enough to put the .dll's where they should go?

[tkelman]

I don't think we want the static libraries at all for any of the libraries that we build using cmake.

[staticfloat]

Done and done, let's see if this works, I'll test it with debuild locally as well.

[staticfloat]

Same result with this change, so I think it's good to go. numbers failure persists.