windows: lots of gtest warnings on windows when building shared libs

[cyrush]

Example:
C:\projects\conduit\src\blt\thirdparty_builtin\googletest-release-1.8.0\googletest\include\gtest/internal/gtest-internal.h(589): warning C4251: 'testing::internal::TypedTestCasePState::registered_tests_': class 'std::map<std::string,testing::internal::CodeLocation,std::less<_Kty>,std::allocator<std::pair<const _Kty,_Ty>>>' needs to have dll-interface to be used by clients of class

For more examples, see:
https://ci.appveyor.com/project/cyrush/conduit/build/1.0.643

I believe these didn't happen with gtest 1.7.

Seems like this issue:
google/googletest#860

This PR may provide a fix:
google/googletest#862

(But this PR hasn't been merged into gtest)

[cyrush]

In the interim, It may be possible to mute these issues by adding extra compiler flags or defines when we use blt_register_library for gtest and gmock when on windows.

[cyrush]

Adding /wd4251 /wd4251 to the compile flags in blt_reg_lib for gtest and gmock squashes these.

This isn't likely as targeted as the solution in the pr for gtest, but I think its a good temp solution. I'll work on a pr.

[cyrush]

closing, resolved in PR #83