build: fix LTO for clang

Firstly clang needs in gold linker, so add a wrapper for it.
And this is not the end, because gold linker needs in plugin otherwise
it can't read object files that generated by $(clang -flto), seems that
it not generate GIMPLE as gcc, instead it have IR (Intermediate
Representation).
This plugin will be added automatically for ld when needed, but not for
ar, so when the archive libs will be introduced we will need to pass it
manually.

For libraries (dynamic and static):
---------------------------------
  (They are not exist for our project for now, but jsut in case)

  If you will have different version of this plugin and compiler, (since
  clang will read only /usr/lib/LLVMgold.so and not look into version)
  you will get segmentation fault for gold linker (see also [1]):
  (gdb) bt
     #0  0x00007f368150e948 in llvm::AttributeImpl::getKindAsString() const () from /usr/lib/x86_64-linux-gnu/libLLVM-3.3.so.1
     #1  0x00007f368150e961 in llvm::Attribute::getKindAsString() const () from /usr/lib/x86_64-linux-gnu/libLLVM-3.3.so.1

  With 3.3 version of clang I couldn't get build to work, because of errors for
  archive libraries and it's symtab:
  - no archive symbol table (run ranlib)
    This was solved by using llvm-ranlib and plugin for ar
    And also if you just use llvm-ar, then llvm-nm will segfault.
  - malformed archive header name at {8,138}
    This was solved by upgrading llvm/clang to 3.4 from experimental
    debian distribution.

And here is some numbers what we have now:
------------------------------------------
  component:      gcc lto       clang lto
  deb package     1.3M          1.6M
  boostcached     4.1M          7.4M

  $ readelf -S boostcached-gcc   | gawk '{if ($1 ~ /^[0-90-f]+$/ && name ~ /^\./) { printf "%s %.0f\n", name, strtonum("0x" $1); } name=$(NF > 3 ? NF-3 : ""); }' | sort -k1,1 >| symtab.gcc
  $ readelf -S boostcached-clang | gawk '{if ($1 ~ /^[0-90-f]+$/ && name ~ /^\./) { printf "%s %.0f\n", name, strtonum("0x" $1); } name=$(NF > 3 ? NF-3 : ""); }' | sort -k1,1 >| symtab.clang
  $ join -j1 symtab.gcc symtab.clang | awk '{diff=($3-$2)/1024; if (diff < -10) { printf "%18s: %.3fKB\n", $1, diff}; total += diff; } END { printf "Total: %.fKB\n", total }'
         .debug_abbrev: -84.449KB
        .debug_aranges: -11.188KB
           .debug_line: -20.646KB
            .debug_loc: -771.084KB
               .rodata: -11.891KB
               .strtab: -32.161KB
    Total: 3359KB

[1] https://retrace.fedoraproject.org/faf/problems/1368472/
travis-ci: install llvm-dev, that will LLVMgold plugin for LTO.