Adding OSX support

[haysclark]

It would be awesome to get this working with OSX. Thus far, I can get Ion to compile; however, bazel fails to compile.

bazel --bazelrc tools/bazel_linux.rc build -c opt seurat/pipeline:seurat                  master 7b20ac6
WARNING: /private/var/tmp/_bazel_hays/374f9938d7a1ef07b0593fc9df44f4c9/external/gflags/WORKSPACE:1: Workspac
e name in /private/var/tmp/_bazel_hays/374f9938d7a1ef07b0593fc9df44f4c9/external/gflags/WORKSPACE (@com_github_gflags_gflags) does not match the name given in the repository's definition (@gflags); this will cause abuild error in future versions
ERROR: /private/var/tmp/_bazel_hays/374f9938d7a1ef07b0593fc9df44f4c9/external/openexr/BUILD.bazel:358:15: Co
nfigurable attribute "defines" doesn't match this configuration (would a default condition help?).
Conditions checked:
 @openexr//:linux_x86_64
 @openexr//:windows_x86_64
ERROR: Analysis of target '//seurat/pipeline:seurat' failed; build aborted:

/private/var/tmp/_bazel_hays/374f9938d7a1ef07b0593fc9df44f4c9/external/openexr/BUILD.bazel:358:15: Configurable attribute "defines" doesn't match this configuration (would a default condition help?).
Conditions checked:
 @openexr//:linux_x86_64
 @openexr//:windows_x86_64
INFO: Elapsed time: 0.113s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (0 packages loaded)

I looked at Ion, and while it is encouraging that it compiles it does seem to have some quirks. For instance, it compiles too /seurat/third_party/ion/gyp-out/mac-ninja/opt, yet no config references the mac-ninja folder.

I tried tweaking some of the files to create a 'mac_x86_64' config; however, eventually, I just hit dependency issues deeper in the build process. It's almost like I want to force the compiler to think it's 'linux_x86_64'.

If anyone has an idea of how to force the build process to compile as Linux, or how to perform the needed tweaks to get the build running on OSX I would love to know!