pass clangcl compile options to ignore warnings in gflags.cc
pass clangcl compile options to ignore warnings in gflags.cc

R=kjellander@webrtc.org, ajm@webrtc.org
BUG=webrtc:760

Review URL: https://codereview.webrtc.org/1426883002 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10440}

TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc