Fix build

[springmeyer]

The build and tests were failing locally for me on OS X. This fixes the core problem:

• The build and tests need environment variables set dynamically
• This is very difficult to do in a Makefile since you can't source bash scripts in makefiles in a portable way on OSX and Linux
• So, the solution is to:
  • Set specific variables like PATH in the Makefile by pre-pending
  • Call out to a bash script test runner that sources multiple variables

This fixed this build error:

PATH="./mason_packages/.link/bin/:/Users/dane/.yarn/bin:/Users/dane/.nvm/versions/node/v4.8.3/bin:/Users/dane/Library/Python/2.7/bin:/Users/dane/.homebrew/opt/coreutils/libexec/gnubin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/opt/X11/bin:/Library/Frameworks/Mono.framework/Versions/Current/Commands" /Applications/Xcode.app/Contents/Developer/usr/bin/make -C build/ BUILDTYPE=Release V=
  ACTION gyp_build_gyp_make_vector_tile_target_run_protoc Release/obj/gen/vector_tile.pb.cc
/bin/sh: protoc: command not found
make[1]: *** [Release/obj/gen/vector_tile.pb.cc] Error 127
make: *** [libvtile] Error 2

And this test error:

PATH="`pwd`/mason_packages/.link/bin/:/Users/dane/.yarn/bin:/Users/dane/.nvm/versions/node/v4.8.3/bin:/Users/dane/Library/Python/2.7/bin:/Users/dane/.homebrew/opt/coreutils/libexec/gnubin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/opt/X11/bin:/Library/Frameworks/Mono.framework/Versions/Current/Commands" /Applications/Xcode.app/Contents/Developer/usr/bin/make -C build/ BUILDTYPE=Release V=
make[1]: Nothing to be done for `all'.
DYLD_LIBRARY_PATH= ./build/Release/tests
dyld: Library not loaded: @rpath/libclang_rt.asan_osx_dynamic.dylib
  Referenced from: /usr/local/lib/libmapnik.dylib
  Reason: image not found
make: *** [test] Abort trap: 6

/cc @GretaCB @flippmoke