BUG: Disable gtest using pthreads on mingw
BUG: Disable gtest using pthreads on mingw

Mingw is using for compiling on MS Windows for R. Compiler errors are
produced when using googletest on the platform:
In file included from

C:/b/GTest-prefix/src/GTest/googletest/include/gtest/internal/gtest-internal.h:40:0,
                 from
                 C:/b/GTest-prefix/src/GTest/googletest/include/gtest/gtest.h:58,
                 from
                 C:\b\GTest-prefix\src\GTest\googletest\src\gtest-all.cc:39:
C:/b/GTest-prefix/src/GTest/googletest/include/gtest/internal/gtest-port.h:1782:3:
error: 'AutoHandle' does not name a type
   AutoHandle thread_;
   ^
In file included from
C:\b\GTest-prefix\src\GTest\googletest\src\gtest-all.cc:43:0:
C:/b/GTest-prefix/src/GTest/googletest/src/gtest-death-test.cc:637:3:
error: 'AutoHandle' does not name a type
   AutoHandle write_handle_;
   ^
Suggested-by: Richard Beare <9650b83a647fecb7525c7eeddaf815961eb53f55@gmail.com>

richardbeare/SimpleITK,richardbeare/SimpleITK,blowekamp/SimpleITK,InsightSoftwareConsortium/SimpleITK,richardbeare/SimpleITK,InsightSoftwareConsortium/SimpleITK,SimpleITK/SimpleITK,SimpleITK/SimpleITK,kaspermarstal/SimpleElastix,InsightSoftwareConsortium/SimpleITK,InsightSoftwareConsortium/SimpleITK,blowekamp/SimpleITK,blowekamp/SimpleITK,kaspermarstal/SimpleElastix,SimpleITK/SimpleITK,richardbeare/SimpleITK,richardbeare/SimpleITK,SimpleITK/SimpleITK,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,InsightSoftwareConsortium/SimpleITK,blowekamp/SimpleITK,blowekamp/SimpleITK,richardbeare/SimpleITK,InsightSoftwareConsortium/SimpleITK,kaspermarstal/SimpleElastix,blowekamp/SimpleITK,kaspermarstal/SimpleElastix,SimpleITK/SimpleITK,SimpleITK/SimpleITK,richardbeare/SimpleITK,InsightSoftwareConsortium/SimpleITK,SimpleITK/SimpleITK,SimpleITK/SimpleITK,InsightSoftwareConsortium/SimpleITK,blowekamp/SimpleITK,kaspermarstal/SimpleElastix,richardbeare/SimpleITK,blowekamp/SimpleITK