premake.lua: relax warnings for GTest and perftest (fixes #86)

For the "test" solution, the "gtest" and "perftest" projects contain
third-party code.  Relax the warning settings for "gmake" for these
projects to avoid build failures due to third-party code.
Merge pull request #93 from pah/fixes/86-gtest-werror

premake.lua: relax warnings for GTest and perftest (fixes #86)