update msgpack library name

[dbanetto]

msgpack library got renamed to msgpackc PR #50534 on homebrew

without fix build fails with:

VIM=/usr/local/Cellar/neovim/HEAD/share/nvim NVIM=/usr/local/bin/nvim scons -Q
WARNING: Could not determine exact Neovim version. If your Neovim is not up to date, there may be incompatibilities.
Install file: "res/Info.plist" as "build/Neovim.app/Contents/Info.plist"
clang++ -o build/client.o -c -std=c++11 -stdlib=libc++ -g -Wno-deprecated-register -I/usr/local/Cellar/msgpack/1.4.1/include -Ibuild -Isrc src/client.cc
clang++ -o build/process.o -c -std=c++11 -stdlib=libc++ -g -Wno-deprecated-register -I/usr/local/Cellar/msgpack/1.4.1/include -Ibuild -Isrc src/process.cc
clang++ -o build/rpc.o -c -std=c++11 -stdlib=libc++ -g -Wno-deprecated-register -I/usr/local/Cellar/msgpack/1.4.1/include -Ibuild -Isrc src/rpc.cc
clang++ -o build/vim.o -c -std=c++11 -stdlib=libc++ -g -Wno-deprecated-register -I/usr/local/Cellar/msgpack/1.4.1/include -Ibuild -Isrc src/vim.cc
clang++ -o build/vimutils.o -c -std=c++11 -stdlib=libc++ -g -Wno-deprecated-register -I/usr/local/Cellar/msgpack/1.4.1/include -Ibuild -Isrc src/vimutils.cc
clang++ -o build/app.o -c -std=c++11 -stdlib=libc++ -g -Wno-deprecated-register -I/usr/local/Cellar/msgpack/1.4.1/include -Ibuild -Isrc src/app.mm
clang++ -o build/font.o -c -std=c++11 -stdlib=libc++ -g -Wno-deprecated-register -I/usr/local/Cellar/msgpack/1.4.1/include -Ibuild -Isrc src/font.mm
clang++ -o build/graphics.o -c -std=c++11 -stdlib=libc++ -g -Wno-deprecated-register -I/usr/local/Cellar/msgpack/1.4.1/include -Ibuild -Isrc src/graphics.mm
clang++ -o build/input.o -c -std=c++11 -stdlib=libc++ -g -Wno-deprecated-register -I/usr/local/Cellar/msgpack/1.4.1/include -Ibuild -Isrc src/input.mm
clang++ -o build/keys.o -c -std=c++11 -stdlib=libc++ -g -Wno-deprecated-register -I/usr/local/Cellar/msgpack/1.4.1/include -Ibuild -Isrc src/keys.mm
clang++ -o build/main.o -c -std=c++11 -stdlib=libc++ -g -Wno-deprecated-register -I/usr/local/Cellar/msgpack/1.4.1/include -Ibuild -Isrc src/main.mm
clang++ -o build/menu.o -c -std=c++11 -stdlib=libc++ -g -Wno-deprecated-register -I/usr/local/Cellar/msgpack/1.4.1/include -Ibuild -Isrc src/menu.mm
gperf -cCD -L C++ -Z RedrawHash -t src/redraw.gperf > build/redraw-hash.gen.h
clang++ -o build/redraw.o -c -std=c++11 -stdlib=libc++ -g -Wno-deprecated-register -I/usr/local/Cellar/msgpack/1.4.1/include -Ibuild -Isrc src/redraw.mm
clang++ -o build/view.o -c -std=c++11 -stdlib=libc++ -g -Wno-deprecated-register -I/usr/local/Cellar/msgpack/1.4.1/include -Ibuild -Isrc src/view.mm
clang++ -o build/window.o -c -std=c++11 -stdlib=libc++ -g -Wno-deprecated-register -I/usr/local/Cellar/msgpack/1.4.1/include -Ibuild -Isrc src/window.mm
clang++ -o build/Neovim.app/Contents/MacOS/Neovim build/client.o build/process.o build/rpc.o build/vim.o build/vimutils.o build/app.o build/font.o build/graphics.o build/input.o build/keys.o build/main.o build/menu.o build/redraw.o build/view.o build/window.o -L/usr/local/Cellar/msgpack/1.4.1/lib -lmsgpackc -lc++ -lmsgpack -framework Cocoa -framework Carbon
ld: library not found for -lmsgpack
clang: error: linker command failed with exit code 1 (use -v to see invocation)
scons: *** [build/Neovim.app/Contents/MacOS/Neovim] Error 1
make: *** [all] Error 2

tested on OSX: 10.11.4

[rogual]

Thank you!