This fixes building of gtest on msvc.
See e.g. http://stackoverflow.com/questions/8274588 for
more discussion on the matter.
This is only required on MSVC 2012 - gtest itself (in the cmake
files) adds -D_VARIADIC_MAX=10 to the build when building using
MSVC 2012, but we don't know the MSVC version at the make stage
here. Since the gtest tuple support isn't used at all, it's
simpler to just disable it altogether than to try to add
-D_VARIADIC_MAX=10.
MSVC doesn't export any functions at all by default, contrary to
gcc which exports all global functions by default.
Doing the same for linux/OS X isn't too hard, but they use yet
two other, different syntaxes for the corresponding symbol file.
This makes sure that e.g. "make clean" actually removes
the built exe files, and avoids relinking the exe files each
time make is run when cross compiling from linux to windows.
(Make on windows seems to have an exception that knows to skip
rebuilding the 'h264enc' target if 'h264enc' doesn't exist but
'h264enc.exe' does, but this exception doesn't work on normal
unix make.)
