Improve warning configuration for clang.
Improve warning configuration for clang.

Clang 4.0 introduced -Wdouble-promotion, so check for and disable
that on clang as we do on gcc.

The old style casts warning is already disabled globally in this file
for clang, so it doesn't need to be re-disabled within this scope.

aimotive/imgui-glfw,bkaradzic/imgui,ocornut/imgui,dougbinks/imgui,wflohry/imgui-addons,tom-seddon/imgui,Pagghiu/imgui,bkaradzic/imgui,mikesart/imgui,dougbinks/imgui,aimotive/imgui-glfw,elect86/imgui,Flix01/imgui,mikesart/imgui,nProtect/imgui,burningreggae/imgui,ocornut/imgui,cmaughan/imgui,ggtucker/imgui,elect86/imgui,Flix01/imgui,ocornut/imgui,tom-seddon/imgui,nProtect/imgui,nProtect/imgui,ggtucker/imgui,burningreggae/imgui,bkaradzic/imgui,Flix01/imgui,Pagghiu/imgui,cmaughan/imgui,ggtucker/imgui,wflohry/imgui-addons,cmaughan/imgui,cmaughan/imgui